I don't know about filler, but for my floors I put down Por-15 first then seam sealed and the seam sealer didn't have a problem sticking to the por-15, nor the top coat to the seam sealer.

As far as prep goes, for por-15 this is what I do:

1. Usually paint stripper or wire wheel down to bare metal.
2. If any rust is present I wire wheel it off.
3. Get metal as clean as possible using the wire wheel.
4. Then rough up with 240-400 grit paper.
5. Then I use their marine clean (not laquer thinner or any other solvent) cut with boiling water and go over the area 2-3 times, let dry thoroughly.
6. Then start with Metal ready, 20 minutes or so till the surface turns cloudy then rinse and let dry.
7. Apply por-15, allow to dry till only slight finger drag.
8. Now a 2nd coat of Por-15 same deal with allowing to dry.
9. Then seal if needed then top coat.

Works very consistent now. I was still apprehensive about doing my floors (since I spent so much time cleaning them up) but I really scrubbed the heck out of them in step #5

Hmm, I didn't wire wheel the parts that flaked, or use hot water on it, so that may be why mine didn't stick as well.

I haven't seen anyone specifically address "smoothing" the firewall.

You can certainly use a little filler over the spot welds and slight waves....but that's not gonna address the numerous holes and that giant seam running down the top few inches of the firewall. Filler will not fix those. Doing the firewall "right" which means lots of cutting and welding... any filler over that now would have to be "mudded" on (bad) and would be over a seam (bad) and would probably crack pretty quick (bad).

My suggestion would be to just clean up the spot welds and smooth those out....and leave the seam alone. Clean the heck out of everything and shoot a coat of satin black and it will look very good. Clean up and detail the wiring and hoses under the hood and it will look better than it came from the factory.

That'll cost ya nothing but some time and labor and a few bucks in paint and it won't come back to haunt you shortly after the car is back on the road.

Filler doesn't work like plaster.... at least not when done right.

-R

Yah, I know mudding the seam would be a no no with cleaning out out and probably welding it up. I've already welded in a plate for the heater hole - going to aftermarket heat/AC - still need to weld up the fan hole and a bunch of small ones. I'm quite good at getting it all even prior to any mud. I just haven't mudded before.

Thanks

Get yourself an affordable HVLP gun and single stage paint....you will be suprised how well you actually can do! And it will turn out a lot better than a spray can.

I would strip the firewall to bare metal first, finish welding up your holes...then move to filler. Probably start with a thin layer of Bondo and then move to a glaze...and finish with blocking out primer, clean it, tack it off and paint it.

There are like a million magazine articles covering this stuff...I think Hot Rod has a good paint article this month. You will be fine, just get a plan together and don't rush it!