High Build urethane primer vs. Polyester primer

P

Porsche 356

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.
 
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 PRIMER 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.
 
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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.
 
Hi, guys. Crash, that is a great synopsis of how you get the car ready for color, so thanks for that. Like you, I have time constraints, but my hours of working on the car are separated by days. My concern is that the epoxy will fully cure (> 7 day) before I'm done metal work (or whatever phase I'm on). When that's the case, can I just hit the entire car with a red pad (preferable to 220 for me since I find it easier) before I shoot another coat of epoxy (or urethane)? Would this be the case for the urethane, too? Thanks so much!
 
Great info as always. Other than taking longer, Is it bad to block with finer grits? It makes me cringe to put heavy scratches in.

I am the guy who made the mistake of not having enough primer on my hood and trunk. Pain in the tail and a waste of time. The rest of the car is good with just a couple coats if epoxy.
 
If we are talking Show Car type work, first prime you need a coarser grit to level your primer. Coarser grits (180 and coarser) do a much better job of leveling. Using too fine a grit will not level as quickly or as well especially if you are a novice. Using too fine a grit to start will almost guarantee you will have waves and probably too much build on the panels. Don't worry about the scratches, 150-180 first round, 180 next round, and final sand (assuming 3 applications) 220 switch to 320 to finish. All dry. You can then do a final wet sand with 400-600 grit.
Collision work is a different story, 320 dry to start then 600 wet.
 
Thanks Chris. I may post some more info on what I am doing in my TR7 thread.
 
I'm coming back to this topic a year later, to say that I have been reading that polyester has better corrison protection properties than epoxy primers.

Not sure how true that is. Particulary Z-Chrome and other products like it. I have spoken to guys in my shop who prime with polyester, and they tell me you can leave the area uncoated like epoxy for months without any signs of rust. Too good to be true?
 
well, hanging around in a shop should not rust.

Its basic environment. My biggest peeve with Epoxy are the guys that want to spray the hood to get it flat black. Yes, SPI has better UV protection than all, but epoxy is just bad for uv. Polyurethane and polyester do provide UV protection, which is why we trust the clear to protect WHATEVER is underneath.

So I would not expect it to be strong or last forever uncoated, but the product itself has better protection to UV than epoxy, so if it is not broken down, there is no reason for it to rust..
 
#1 epoxies are not all created equal so to make a blanket statement that polyesters provide better corrosion protection than epoxies isnt right at all. i will say this though, if it provides better corrosion protection than epoxy why dont they paint ships and bridges with it? epoxy is the only thing being used in high corrosion environments.
 
The best way from what ive read is to do 2-3 coats epoxy, let sit 48 hours at 70* F then put poly on. Block poly then go from there with everything else. Started to watch a YT video where the guy chipped poly off of a car where they put poly over bare metal. Rust underneath poly. I've never used poly so don't know from personal experience. I do epoxy, body filler, more epoxy, then a sealer coat of reduced epoxy before paint. Never had any problems.
 
#1 epoxies are not all created equal so to make a blanket statement that polyesters provide better corrosion protection than epoxies isnt right at all. i will say this though, if it provides better corrosion protection than epoxy why dont they paint ships and bridges with it? epoxy is the only thing being used in high corrosion environments.

What did I write about corrostion protection? Didn't I specify SPI as different. How many ships have you painted?

All I mentioned was UV. I wont argue about below water surfaces where there is no sunlight, but not above. that is why they are referred to bridge lines and you know if they used poly they would have to scuff the entire thing before it broke down not just paint over it again. You must be referring to the dull powdering gray ships that get repainted every 3 years or the yachts that use polyurethane? I know what they paint yachts with, we are required to paint the surveillance equipment with the same Awl Grip coating they use on those.
 
They sell polyester primers with zinc chromate in them, that have to be applied to bare metal. They etch into the metal to seal out moisture.

The argument is that, polyester primers will not absorb moisture just like epoxy primers. My friend who owns a bodyshop told me he's been painting with zinc polyester primers since 1990, and has never had one customer come back.

Not sure honestly what I would trust, but I do know that you should buy a quality brand. That part about UV protection is great info, did not know that.
 
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That is all I am trying to share. Last time we got into this black epoxy hoods Barry said he felt they were good for 5 years. I am about to spend 2 years and some 15-20K on materials to do my chevelle, the only reason I want to repaint this is some asshole keyed it, or the color is not doing anything for me anymore.

Thats why there is no problem with anyone wanting to paint their chassis suspension parts with epoxy. Any area the sun dont shine are safe from the epoxy breaking down, powdering and looking terrible. You can see that wheel wells do get some reflection and eventually would go two tone.

All the poster mentioned was no surface rust on polyester. Epoxy is great, but it is as old school as wash primer and eventually will fade away like leaving it in sunlight.
 
I'd put a good epoxy over ANY polyester in a 500 hour salt spray test, any day of the week. There'd be no reason to use epoxy if its adhesion and corrosion resistance were not superior to anything else, because it's slow drying and relatively low building. Who would volunteer for that if it wasn't necessary? Guys who use the hybrid products are using them because of convenience, they build more and dry faster. If they think otherwise they are just kidding themselves. That's not to say the hybrids aren't good enough for general use, but it's seriously laughable to think they could be better than a high quality epoxy.

And yeah, epoxy should be topcoated for best results. So what? This thread was about corrosion resistance, so let's not derail it or muddy the water by pulling in another issue. It's a topic of its own, imo.
 
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