What 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.
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.
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 many 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.
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.
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 and you will have to sand it for the paint to stick. Sealers do not require sanding and acts as an adhesion promoter for the top-coat to stick to.
Primers are porous and are not really designed to be a sealer per-say. Primers are designed 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.
Sealers are not porous and what that means is that it will resist absorbing water or moisture whereas primer can absorb moisture and get trapped in the primer. A good 2K sealer creates a water 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.
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 always 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.
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.
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 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.