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Re: Surface Rust

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Duane WI

07-09-2013 18:10:13

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My understanding is that epoxy primer needs some extra surface scratches to mechanically lock into. Most people seem to recommend sanding with 80 grit before epoxy. I would think the rust pits have jagged sides and the epoxy will lock into them like a cork in a bottle. Doesn't sand blasting actually smooth out the metal surface? This is a good discussion I am learning as I go. Thanks

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Rod (NH)

07-09-2013 19:13:40

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 Re: Surface Rust in reply to Duane WI, 07-09-2013 18:10:13  
If you do a bright-metal blast using standard Black Beauty grit (not sand) there will be no rust remnants left in any pits. The resulting blast profile will be more than sufficient for priming directly with epoxy without sanding. Two coats of epoxy will fill the profile well.

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07-09-2013 18:43:49

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 Re: Surface Rust in reply to Duane WI, 07-09-2013 18:10:13  
The PPG Omni epoxy's tech sheet says no finer than 180 grit, not 80 grit. Used by itself 80 grit scratches may show through the topcoat if no surfacer is used. I usually use 150. I have never noticed epoxy lifting on sheet metal or cast. If there is a nick that removes the topcoat, the epoxy usually stays intact, that is what it is supposed to do. For hoods, gas tanks, and grilles, the initial paint and rust removal, the fiber wheel I use is much coarser than 80 grit. I attempt to flatten it by using 100 and 150 grit, then I apply epoxy followed by surfacer. If I look closely I can sometimes see minor scratches through the topcoat. For things like battery boxes, seat frames, etc, which is smooth metal, but not sheet metal I use 150 grit and epoxy with no surfacer, any light damage doesn't chip the epoxy there either. As to sand blasting, I sand it afterwards to smooth it out and also to remove any sand that may (or may not) be imbedded in the metal. However it is probably smooth enough to begin with -- I just like a smoother surface.

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Duane WI

07-09-2013 19:09:47

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 Re: Surface Rust in reply to CNKS, 07-09-2013 18:43:49  
This is an old pickup so I plan on using high build primer and block sanding it out. That is after I get all of the small dings and dents filled with body filler. I have most of the dents worked out pretty well with a cheap set of body dollies and hammers. My goal right now is just to get all of the metal work done and get everything in epoxy primer. I also want to get the frame cleaned up and painted this summer.

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07-10-2013 06:39:06

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 Re: Surface Rust in reply to Duane WI, 07-09-2013 19:09:47  
Remember that epoxy has a window -- about 3 days to a week,(depending on the brand) that you have to apply topcoat or surfacer, etc on. Outside the window you have to scuff/sand the epoxy and apply another coat -- scuffing without another coat is not enough to make the epoxy bond to the paint. As to high build primer, urethane surfacer (uses hardener) is best because it doesn't shrink and expose the scratches.

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Duane WI

07-10-2013 06:58:08

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 Re: Surface Rust in reply to CNKS, 07-10-2013 06:39:06  
My plan has been to scuff the epoxy and apply the body filler and sand it out. This will be after the epoxy has fully cured. Could be next summer before I get to it. Then apply another coat of epoxy followed by the high build primer. Will the body filler stick to the cured and scuffed epoxy or does it need fresh epoxy? Also does the high fill primer need to be sealed before final top coat? Again thanks for the help.

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Rod (NH)

07-10-2013 11:33:37

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 Re: Surface Rust in reply to Duane WI, 07-10-2013 06:58:08  
Here's what I have done successfully in a situation like that: I like putting epoxy on almost immediately after blasting to bright metal. I do all my work outside in the summer and live in a fairly humid area. Even fingerprints on a bright-blasted steel surface can start the rusting process. Getting epoxy on as soon as possible protects the metal for additional work later on. I've also used Picklex 20 for that purpose but prefer epoxy if I can accomplish it at the time. If there are some areas that need plastic filler later on (hammer/dolly work should have already been completed), I'd rough the area with a course grit (say 80). That will go to bare metal in the heavy scratches. I then would apply a fresh coat of epoxy to the area and apply the filler within the epoxy window. After sanding the filler as needed, there may still be spots where sanding went to bare metal. Scuff the area at that point (Scotch-Brite is good for that) and apply a final coat of epoxy. You then could go directly to the topcoat within the epoxy window if the surface is smooth enough at that point or apply a few coats of surfacer. You then could delay the topcoat for a time of your choosing without worrying about any further time windows. You'll need to final sand the surfacer anyway and that will provide the "tooth" for the topcoat.

I have never had a problem applying filler over epoxy that way. I'm not questioning that filler to bare metal with a heavy scratch is "better" - and I've seen some test data that so indicates. However, I remain unconvinced that it's necessary in the vast majority of cases. So I prefer epoxy first, then filler.

I know that B- prefers to add a fresh coat of epoxy between the final-sanded surfacer and then topcoat within the window. Not as any kind of sealer - no sealer is required. The coat of epoxy at that point provides for a chemical bond with the topcoat and is "better" than just the mechanical bond using only the scratches from the surfacer sanding, usually with a final 320 grit or so. I've never had a problem skipping that step and feel that while it is "better" it's likely not necessary considering the more applications you do, the more chances for something messing things up.

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07-10-2013 08:43:06

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 Re: Surface Rust in reply to Duane WI, 07-10-2013 06:58:08  
Some controversy about your question. I like body filler on bare metal, some like it over epoxy, probably doesn't matter, but I put it on bare metal. As far as fully cured, epoxy can have something over it in 30 minutes, so curing doesn't matter as long as it is in the window. But, I can't answer whether or not the body filler sticks to old epoxy or not--I would sand it first. There is a good chance when you sand that part/all will go to bare metal. Rod(NH), far as I'm concerned, along with Glennster, B-maniac and Gordo and a very few others on this forum are the experts on epoxy--I have my own opinion which sometimes disagrees, but one of them will give their opinion. With PPG products you do not have to seal the high build primer, some other brands may say you do. Some say that applying epoxy over the surfacer, etc makes the topcoat adhere better--that is a possibility, I have not tried it.

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