What is the difference between primer and sealer on a car?

What is the difference between primer and sealer on a car?

What is the difference between primer and sealer on a carWhat is the difference between primer and sealer in the automotive paint industry? This is a great question and could lead into a forever explanation but I will make it as short as I can.

Primer is basically a material to make something ready for the next stage. Sealer is pretty much the word, it seals something but in this instance, we are not just sealing something in but also sealing something out.

Dupont Uro Primer 1140SWhat Is Primer?

Most primers today are urethane (2K Primers), they are great for building layers you can build up to block sand down and get a more wave-less surface than you could with the old lacquer primers . Being a 2K urethane product which 2K just means it is catalyzed, this could serve as a sealer. However, primer is mostly used to spray over damaged and repaired areas to create a level and protected surface. Nason makes a really great Urethane Primer, easy to mix, spray and sand.

In order to create that level surface, it means you have to build up your layers of primer by spraying several coats and then block sand the primer down smooth and level. In how to wet sand a carmany cases, you are going to cut through the primer on the high areas of an unlevel surface and even down to metal on some edges.

Now you could go back and prime again and then do a medium sanding with 400 grit or you could spray a 2K sealer that is designed to bond with a top-coat. In other words, no more sanding, the sealer is used to protect the top-coat from areas where you have sanded through the primer and most sealers are colored for maximum top-coat coverage.

Why Color Your Sealer

When you buy automotive paints, the paint store should tell you what color sealer to use so that your paint will cover better. Imagine trying to spray white over a red surface, it is going to take much more paint to cover and to look uniformed.

Sealers come in more colors than primers do, which works great in what they call the “shading factor”. What this means is apply a shade that helps your top-coat cover. At one time you could by sealers in colors but now they have figured that shades like white, grey, black and dark grey will enhance coverage. I have been using Cromax for 2K sealers and their three different shades with great results.

Why Not Use Primer For A Sealer?

Primers are porous and are not really designed to be a sealer per-say. Primers are primered surfacedesigned to be used to fill mild waves, scratches and put a protected layer between bare metal and your top-coat.

Paint can be applied over primer but if you would like to increase the life of your paint job and keep from having any surprises pop up on you down the road, a good 2K sealer is your best bet.

So you do not have to use sealer if you are using a 2K urethane primer but be prepared to use more of it so you have an even uniformed surface. You will have to sand the primer to promote adhesion for the top coat but if you sand through the primer, you will need to apply another coat and sand that coat. Sealers do not require sanding and acts as an adhesion promoter for the top-coat to stick to so this makes it easier if you sand through the primer in places but then coat it with the sealer.

The Benefits Of Sealer

Sealers are not porous and what that means is that it will resist absorbing moisture whereas primer can absorb moisture and get trapped in the primer. A good 2K sealer creates a moisture tight sealed barrier between the prepared surface and the topcoat.

Sealer also acts as an adhesion promoter that adheres to the prepared surface like a glue but also does the same thing for the top-coat that is sprayed on it.

The most common sealers are the non-sanding types, you apply it and it becomes a uniformed in adhesion and color layer to enhance your top-coat.

 

Use A Non-Sandable Sealer

There are sealers that require scuffying or sanding and I do not recommend these types. I have quite a few reasons for not liking these but in summary, wet sanding or scuffying causes debris build ups in cracks and corners and paint will fisheye and or not stick like a sealer will.

Anytime you do your final wet sand on a car before painting it, you want to so a good detergent wash and ensure that all the nooks and crannies are cleaned out. Sadly there car with sanded primeralways is some left here and there, a sealer and primers for that matter, tend to melt in these places and create a bondable area.

So a non-sandable sealer will create that layer for bonding. If you so get dirt in your sealer, you can always nib out the dirt with a small piece of sandpaper or scuffy pad.

Can You Spray Primer Straight Over Metal And Fillers?

Even though technical manuals and or auto body teachers say apply a urethane primer straight over metal and body filler, I recommend spraying an etching primer first. Not a thick heavy coat but a light to medium wet coat. Scuffy it after it dries and then apply your coats of urethane primer.

My Recommendation

I recommend using sealer because you will use less paint and get a longer lasting job. You do not have to sand it and a good sealer is design to promote adhesion between the car surface and the paint. So that is pretty much the difference between primer and sealer on a car.

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14 thoughts on “What is the difference between primer and sealer on a car?

    1. You do not want to wait more than an hour on most sealers. I think many say up to 2 hours. If you wait too long, you will have to use a scuffy pad to rub the surface before applying the paint. If you spray your basecoat within an hour, you should be OK. It is important that you ask your paint supplier or read the specs on your sealer as many are different but a rule of thumb that I use is between 30 minutes and an hour. Let me know if that helped. The sealers I use are 2K and I usually try to get the basecoat on between 30 and 45 minutes.

  1. Thanks for this article, helps a lot. Been struggling to wrap my head around this. So in the cases where you use primer, you recommend following with sealer after leveling the primer?

    When can you get away with just using base coat? In instances when you don’t sand through the original base?

    Thanks!

    1. These are great questions, I will try my best to explain. For the best long lasting job you can get, you will always want to use sealer and here is why. Paints, regardless if it is a base coat clear coat job or a single stage, they have solvents in them, partly to thin it and part what they are made of. These solvents will basically soak into the old paint or primers, depending on what surface you are spraying over. Once the solvents start soaking in, they will lift the old paint or primer almost like paint remover. Sometimes this process can happen instantly or take up to a few months. But you will see it and this can start peeling and flaking.

      Primering is mainly used for leveling a surface and fixing blemishes but it too can help seal the surface underneath. In the old days, primers were mostly lacquer, which enamel paints laid on top really well as enamel solvents cant cut lacquer. Although you will find all sorts of information saying I am wrong but to me, the proof is in the pudding. You can reduce enamel with lacquer thinners but you can’t reduce lacquer paints with enamel reducer. You can rub a rag soaked with enamel reducer all day over top a lacquer finish and it won’t hurt it but take a rag full of lacquer thinner over an enamel painted surface and see what happens.

      Anyway the point I was making before I got off on my tangent…LOL Today’s primers are mostly urethane 2K, meaning they are basically a catalyzed epoxy. So a urethane primer can act as a sealer and if all you are going to do is lightly sand it or just scuffy the surface and not cut through the surface of the primer back down to the original surface, you could paint over this without using a sealer. The main reason for using a sealer over top a primer is, usually based on the idea that you are going to sand the crap out of the primer to get a level and slick surface. And in that process you are going to cut through to the original surface in many areas. So you spray a catalyzed sealer over all the work you did and you will get a long lasting and uniformed job.

      The one issue you will have with using primers as a sealer is that they do not come in shades. It has become ridiculous these days, but we pay almost 400% more for automotive paints now and sadly they have become thinner and don’t cover worth a crap. So to solve this issue, they make sealers in shade factors so that your base coat covers better. Usually the shades run as follows and this is the basics: White, Light Grey, Dark Grey and Black. Depending on who the paint manufacturer is, there may be more shades than this or less.

      I hope that helps clear up some of the confusion, if not contact me through the contact page here on my site.

        1. More than happy to help, sorry my explanation was so lengthy but I wanted to write something others can read as well and for it to help them.

  2. I have a VW beetle 1972, think it has two coats of primer then it was sprayed with car wrap, that’s is like a rubberised paint. I have managed to remove this and suspect the two coats of paint remaining are cellulose. What should I use to seal the surface before I use a cellulose base colour?

    1. I am not real familiar with cellulose but from what I remember isn’t is lacquer based? I hate that plasti-dip rubberized coatings, guess it would be OK if you are planning a bank robbery and wanted to change the color of your car real quick, otherwise that stuff is expensive and really a waste of time. If you are going to hassle with spraying it, just paint it for real.

      Anyway, I would sand the two coats of primer down, meaning block sand it really good so you get the surface level and slick. Plus this will give you an opportunity to ensure there is nothing wrong beneath the outer surface of the primer. If all looks well and you see no signs of primer bubbling or possible reactions where the primer went over the original surface, I would get a 2K sealer and apply one good medium wet coat. Then spray it with your cellulose.

  3. I am confused, are you saying not to use a “primer” and use the Cromax instead? As in repair with epoxy, filler, Cromax, paint, clear coat. Or are you saying do the repair with epoxy, filler, primer, Cromax, paint, clear coat?

    1. No, I am not saying that. For every type of job or repair, there are always circumstances that may require doing things a bit different from each job. On some repair jobs where you will be blending the paint in a smaller area, let’s say a bumper corner. You would repair the dent or scratch. If the bumper was metal, I would fix the dent, feather sand the area around the damage and then use filler. Sand and possibly fill again until it looks right. Then I would use a urethane primer. I would sand it until it all looks smooth, even and kind of blended in with the old surface. Now here is where the question comes in, to use sealer or not. If I haven’t sanded through the primer anywhere and the base color calls for going over a light gray such as the primer color, then I would not use the sealer. If the color calls for a lighter or darker shade than the gray primer, then I would lightly cover just the damaged area with sealer. Then spray by base coat, applying the needed coats to cover the damaged area and fanning out my color on each pass. Once I feel the color has blended out and everything looks uniformed, I would clear the entire bumper.

      If you are paining an entire car, fix and repair any dents and scratches. Use filler over dents if necessary. Using a urethane primer, prime the the areas that had damage where you filled or had to sand a lot. Block sand the primer areas, if you sand through the primer areas mildly, then you can leave it and spray sealer over it when you spray sealer over the entire car. If you have to sand most of the primer off to get it level and smooth, shoot another coat of primer over the areas again, then block it one more time. Once you are done leveling and smoothing with the primer, spray sealer over the entire car, spray your base coat and then two or three coats of clear.

      If you are repairing a plastic bumper cover, then you would pop the dent out if it had one or if it is a mild damage, sand the damaged area out. If the bumper cover has a mild gouge, scratch, dent or wave in it that might need some filling. I would recommend sanding the area out as smooth as you can get it, then apply at least tow coats of a urethane primer over it. Then apply a flexible filler designed for bumper covers. I would sand out the urethane primer with at least 100 grit paper before applying the filler. Then work your filler like normal and then follow the steps above.

      I hope that clears up how to use primers and sealers.

  4. Question…I’ll be spraying a Medium Blue Acrylic Enamel Metallic. Will a lighter shade of sealer like light gray make the end result a lighter shade of blue as opposed of using a dark shade of sealer. I do not want to lighten up the color i allready have. If anything i want it a little darker than what it is. I know on base/clearcoat system it will effect the final outcome. Not sure with Acrylic Enamel.

    1. When it comes to shades of your colors, you do not want to rely on the sealer color to determine your shade. For the main reason is if you ever had to do a repair, it will make matching impossible. The other reason is it will make it harder to make the paint job come out even when shooting the entire car.

      You want to ensure that the color of sealer is what is called for for that base color. Your paint store should tell you that. However gray sealer sounds correct for medium blue. Getting the correct color shade of sealer just cuts down on the number of coats of base color you would need to spray to get the best even coverage without having to apply many many coats and cause you to have to buy more paint than what you should need. If the gray is the correct color shade, then you shouldn’t need more than 3 coats of base color.

      If you want to change the shade of you base color, either take it back to you paint store and have them tint it or get some tint from them so that you can play with it on a test panel until you get the shade you want. But like I said, do not try to play with color shading using the color of your sealer, that will be disastrous in the end job and any possible future repairs.

      Your method would be like shooting a tri-coat paint job. If you have seen cars with white with pearl paint jobs, you are looking at a tri-coat job. You can’t mix pearl into white and still see it. So you have to lay down so many coats of white and then so many coats of pearl and then clear. The issue with these nice looking jobs are when you have to do a repair. They do not tell you how many coats of white or pearl they used, so you have to guess and most often it will not match. I hope that made since, if not contact me through the contact page on this site and I will try and help you further.

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