Primer or not?

[trackster]

Going on vacation this Friday for a few days in Gatlinburg.  When we get back I'm going to paint the chassis on the 39 Chevy.  Not looking for a Riddler type look, but want it to be presentable.  Going with PPG epoxy primer on the bare metal,  Concept single stage for the top coat.   Do I need to use a filler primer such as K36 or K38 over the epoxy?   First time painting and want to do it right.  Thanks.

[Digger]

That's going to depend on how smooth the chassis is with the epoxy primer. If it is showing a lot of pits you will probably want to use something like K36. Otherwise just go ahead with the topcoat over the epoxy. or just use gloss epoxy primer and forget the topcoat.
I usually use Imron for chassis paint for its duability.

[Harry]

Ridler...with one D.

[Beck]

I haven't seen any '39 frames that don't have pits. You are in the rust belt too I see, so double the pits. Primer won't make the frame stronger but it will hide the smaller pits. In the end it all depends on how pretty you want the underside to be. You have to remember once you put the primer on you have got to do the sanding. Without block sanding spraying the primer was not worth the trouble or expense. If you want it slick one priming/sanding probably won,t be enough. If you make it pretty you may want to spray body color on the frame. If you go with just the epoxy primer you may want to go with black. If it is black under the car it is dark, like I can't see in the dark. On top of a car black would make the pits obvious imperfections. Under the car it sometimes makes everything hard to see, hiding some imperfections.
my 2 cents worth...

[idrivejunk]

Assuming that any rust has been converted or sanded away-

If you don't need any fill, go color right over the epoxy "wet-on-wet".

If you have only a few rough spots that show, use filler or glaze putty before epoxy and color, and put an extra coat of epoxy (you will probably want two wet coats over the whole thing, plus that) over the filler before topcoating.

If you have slight overall roughness you could use a primer/sealer in between epoxy and color, for up to a couple coats more fill.

The above suggestions limit the additional sanding needed. If you have many large rough spots that are a visual concern and they are going to look bad painted over ... well yeah you better prime at least the bad spots. But that means laying the epoxy first, then filling pits and priming over the filler, then sanding the whole frame again to prep for color.

A polyester primer might be a better choice than a urethane if filling deep rust pits. The material is cheaper and fills more so you might get away with a single application.

[papastoyss]

If the car is a driver I would prime & top coat w/ Rustoleum & call it done. If it's a show car the previous posts are the way to go.

[UGLY OLDS]

I agree .. I use Rust-o-leum on all of my fat fender car frames ... I learned long ago that if someone is in a position to critique the bottom of my car , they are also in the perfect position for me to answer them ....  

It also makes clean-up soooo much easier at the quarter car wash .....

Bob ......