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High Build urethane primer vs. Polyester primer

I know that SPI dose not make polyester primer. However, I would like to hear both side of the argument. I have been online looking at the pros and cons of high build urethane and polyester primers. I have used polyester primer ( Clausen's sandy primer) in the past and I like the way it fills and works but some people say it will crack. I imagine it will crack if it is too thick. On my current two project Porsche 356 cars I would like to know what may be best to use, polyester or urethane and why. I would like to hear from people that have used both.

Thanks for your input, Jim
you generally use both. think of polyester like sprayable body filler. you can put that on heavy then 75% of it gets blocked off. after that your urethane primer goes down. usually if you use polyester then you dont require too much urethane. 2 coats maybe to fill scratches and fine tune the surface. you dont want to use just polyester and put your basecoat directly on it. either a sealer or urethane primer is usually required.


Trying to be the best me, I can be
Always use Epoxy for your base, over bare metal, fiberglass etc. Epoxy will give you the best corrosion protection and adhesion bar none. SPI Epoxy is the best out there. You can also use the SPI Epoxy to build. It doesn't build like a Urethane but if your body is close in metal it would be the best choice. As for your question, it really is a matter of preference. Urethane will build nicely. Poly will build even more than Urethane, but lots of people abuse it because it builds so well. Get your work as close as you can in metal,/filler and don't abuse either one and they will last a log time, especially if you used epoxy as your base. Many people make the mistake of priming then blocking with too fine a grit and reapplying more primer, then repeating that process over and over. Ideally you want to put 3 coats of urethane or 2 coats of poly on to start with. After it dries (give it plenty of time, let it sit in the sun a day or two) you would want to block it with a fairly coarse grit. Anywhere from 80-180. If you are a Novice I'd recommend 150-180 grit. Block until you start to see metal poking through in places. Concentrate on not letting prime build excesively on edges. If you have lows use some 2 part glaze to fill them, sand then re apply Epoxy to your bare metal areas, wait the appropriate time and apply your choice of poly or urethane again. Contimue that process till you have all your lows blocked out. It should block out in 3 applications of primer. 2 would be better but 3 is ok as long as you are sanding evenly and concentrating on not letting primer build on the edges of panels.
Chris and Jim are giving sound advice, as usual. There are many variations on the theme. Here's one timeline that included poly primer, in this case it was a fairly badly damaged GTO fender:

Strip/blast, epoxy, metalwork, sanding (red scuff/220), epoxy, filler, blocking (40/80/150/220/red pad), epoxy, polyester glaze/putty, blocking (100/150/220/red pad), epoxy, poly primer, blocking(100/150/220/320/red pad), epoxy, urethane = ready for final blocking.

These sessions stretch out over weeks for me, since I only get 1-3 hours to work on project cars every several days. Beyond their essential purpose of protecting bare metal and porous filler, the semi-gloss layers of epoxy help show by reflection some subtle things that might be missed with the hand, they also obviously add mils for blocking.

In the method I use for restoration body work, epoxy and polyester filler/glaze/putty play by far the largest part in the straightening process, and the polyester primer step is only needed for seriously damaged panels. In most cases we would skip that step, or replace it with either an additional epoxy or urethane step.

The drawback to poly putty in my mind is it can produce some serious build in areas where it's not wanted, like door handle depressions, side marker holes, edges, etc. Care must be taken to remove the film build from these areas, which has to be done with every other kind of primer, too, but with poly it just seems a lot harder to get it back down to where things will fit right and edges won't chip off.
Alot of great info posted.

Another thing to keep in mind is that when spraying poly primer over fresh epoxy, fresh epoxy needs a longer cure time before poly primer. I think 48 hours cure time is something Ive used and read as a guideline. If using urethane I think its generally accepted that an overnight cure on fresh epoxy is good.
Good point about wait times. It depends highly on surface temps and mil thickness of the application, too. Since buying a 5KW portable electric heater for my spray booth, all temp related problems have disappeared. We can even apply poly primer over 1 coat of epoxy in the same day, if surface temps have been held at 80°+ for a few hours.