Is brush-on primer on steel panels supposed to be soft?

Question

I'm prepping replacement rocker panels (Nor/Am Canada SKU:0897-100L) before installing the panels on my 2010 Silverado.

The panels are bare steel, so I've painted them with brush-on Tremclad Primer before painting them white. I cleaned the steel with brake parts cleaner prior to painting with primer.

It's been 48 hours since I did the painting. I've noticed that the primer is very soft; I can scratch it with my fingernail down to the bare steel.

Is that expected? Does primer dry quickly but take many days to fully harden/cure?

I ask because the soft paint isn't inspiring a lot of confidence when it comes to stone chips and salt spray. If I can scratch it with my fingernail, then stone chips will easily scratch the paint and cause rust. I know with automotive applications, paint is normally sprayed, not brushed. But I went with brushed since I thought it would be thicker and more durable, and the truck is just a beater, so aesthetics are not important (the paint won't be visible from outside of the truck). But now I'm wondering if the brush-on primer is not up to the task.

Answer 1

Oil based paints dry by evaporation of solvent but cure by oxidation of the oil "vehicle". Drying may take a few hours but curing is a prolonged process.

Oil based paint is typically mostly cured within about a week, but full curing can take a month or more. If it still smells like paint, it's not fully cured yet.

Higher temperatures and good air circulation help to speed curing time. If you have hot sunny weather where you live, hanging your parts in the sun would promote faster curing.

In the bad old days of high VOC oil based enamel paints, newly painted cars would be passed through a heated tunnel to speed curing.

Take note that an extra thick coat of brush-on paint will take much more than twice the curing time of two coats of half the thickness. This is because the oil's reaction with oxygen depends on diffusion of oxygen through the full thickness of paint. A quick cure on the outside of the paint layer slows down the diffusion. The thicker the coat, the slower the diffusion. An extreme example is when a skin forms on the liquid surface of a can of oil based paint that has been opened. This protects the main body of paint from curing, potentially for years.

If quick curing is your goal in a future project, two or three coats of slightly thinned oil based paint applied a few days apart should give a much quicker final cure than a single thick coat.