Sheet metal - should MP182 be used between 170 an

Since then I learned on this forum that MP 170 epoxy primer is a superior primer coat, and that"s what I"m using on my current (340U) project. However, while I am able to get a smooth coat, it is not perfectly "flat", and certainly nothing like the smoothness that you end up with after several stages of sanding of 182 primer surfacer. And of course 170 is not very sandable. I"m done with the cast, and it looks fine. It"s the sheet metal I"m concerned with. I"m ready to begin topcoating, and I"m concerned I won"t get as good a final surface when I apply the topcoat MTK.

How do you guys do it? Even if you are not trying to fill pits or imperfections, do you still apply 182 over the 170 prior to topcoating, just to achieve a better surface? Is it possible to do as well using 170 and MTK, without the 182? (that is, is my technique of spraying the 170 the problem?

Thanks

Yes, it's possible to go directly from MP170 to MTK topcoat on clean sheetmetal with no bodywork and essentially no sandscratches. The 170 is a non-sanding primer and doesn't require sanding. However, to do that the 170 has to lay down nice and smooth. If, for whatever reason, it doesn't you can apply some 182 surfacer to fix the problem. You can also try reducing the 170 up to 10% with acetone to try and avoid the extra step(s) with the 182 but I have never tried that. That has to be done beforehand and not after the fact though. There certainly is no problem adding the 182 other than being another step in the process. In fact, the 182 is not really intended to go on bare or even treated metal, although an etch primer is an exception.

The only possibility on my AC-B sheetmetal that didn't require either heavy sanding or sandblasting was the steering column. The old one had a rust out in the bottom. It's just a piece of straight steel tubing so I decided to replace it rather than patch the old one. A length 1-1/2" electrical metallic tubing (EMT) was a perfect match. Since the tubing was new, it didn't require any bodywork or sanding. I just scuffed it with a Scotch-Brite pad, applied 2 coats of 170 and topcoated with MTK directly. In that instance the 170 layed down nice and flat, did not require any sanding with 400 and the topcoat came out nice and smooth and glossy. I am not saying I could do that on a consistent basis - only that it is possible.

So I would say go ahead and use the 170 with the 182 on top before the MTK. That's how it would be done anyway if there were sandscratches. The 182 sands easily with 400 and you'll be assured of a nice smooth surface for the topcoat without relying on an essentially perfect application of the 170.

Rod