I'm prepping the firewall, fenderwells, and core support for paint. For the under-hood area I'll be using single stage PPG.

As I'm sanding some of the scratches out, some of the areas are now bare metal. I know that bare metal should have an etching primer applied, but is is safe to spray it on the entire area, or will the etching primer "attack" the areas still painted?

I would use an etch primer on the bare spots, no point in spraying over the paint. Although myself I could probally use a high build primer to make sure all the scratched and stuff were filled and wetsand it before paint.

Yeah, I am planning on a hi-build primer, I just wanted to make sure the etching primer wouldn't hurt the other areas... even if I just "spot" the bare metal areas, the etching primer will be going over some of the old paint...

Why not kill two birds with one stone and use an etch/fill primer? That's what I have been using all along on my car over body work or bare metal with no problems.

Only use etch primer on bare metal, keep it off filler and old paint.

Spot it lightly on the bare spots, lightly. Don't need much.

Epoxy would be a option also, skipping the etch depending on how much bare metal there is.

I'm looking at PPG DPLF epoxy, and it says on their site that it should NOT be used over an etching primer, but that a metal conditioner should be used first.

PPG DP40LF Product Sheet

Looks like this may be the best way to go, for what I'm doing.

Only use etch primer on bare metal, keep it off filler and old paint.


Have to disagree with you there. I've been doing it for years and never had an issue. Besides there is always going to be some incidental contact between bodywork/filler and bare metal when using etch primer so what does a guy do in that instance? Tape off the filler spots?

For small projects and sand throughs I use Sherwin Willams GBP 988, it comes in aerosol cans and works great,

http://www.sherwin-automotive.com/products/show_product.cfm?product=988

If you're prepping engine bay, I'd avoid hosing on unnecessary materials,by doing so,you will cut down on chances of chipping and scratches, you can use GBP 988 over any surface/substrate without any problems, it has amazing adhesion properties and has never failed me, I use it quite often.It goes on nice and smooth with no builup,dries within a minute or two, I simply love this stuff

Simply shoot one or two medium coats of GBP 988 followed by one coat of tinted sealer and then topcoated with your choice of color and clear.