Removing Primer?

[88whitemanche]

Hey guys

What's the best way to remove primer? When I bought my truck it was "saposebly" primered to paint but never got to that project yet.... I've been noticing on the bed rails that bits of rust spots popping through the primer...I wanted to grind it down but I wanted to make sure with guys with experience here....is there a best way to remove primer?

[Akula69]

Depends upon the type used. Most all will sand, but enamel-based primer will gum the paper up faster. Urethane-based primer/sealer sands easier. Laquer-based primre can be removed with laquer thinner (but makes a mess).

 

All-in-all? Sanding (preferably with a DA) is the way to go with 100-120 grit. Follow up with an urethane-based primer/sealer.

[88whitemanche]

Well to be honest with you I don't know what type based primer he used.... Do I still sand even if I want to take it all off?

[Biotex]

Anywhere there is pitting or rust showing through the primer, you need to sand it down to bare metal with something like 80 grit. This will dig into the bare metal just enough so the new primer, or bondo will have a good mechanical bond.

 

The rest of the primer just needs scuffed up with a 3M scotch-brite pad. If the primer was applied correctly, it won't come off. If it does, then you should think about sanding down the whole car and doing it right.  If you properly scuff or sand the primer, then the new coat of primer/sealer you lay down just before the first coat of paint will also have a good mechanical bond.

 

You see, there are two types of bonding. Mechanical and chemical. The chemical bond is what is known as the recoat window. Your primer/sealer will have it listed something like 12 hour recoat at 72°F. This means you need to plan on applying the first coat of paint within this time frame to get a good chemical bond.

 

The primer already on your truck is obviously long past the chemical bond stage, so you are after a good mechanical bond. This is where the scuffing comes into play.

The primer has been on long enough, that it is bonded both chemically and mechanically to the metal at this point, so no real reason to remove it all. Just in the spots that you mention were the rust is coming through.

 

With that said, I'm assuming  you are talking about a quality automotive primer, and not some rattle can lacquer based primer. If that is what is on, then I would consider using a stripper in the hard to reach areas, and a DA on the flat surfaces.

Hope this helped.