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body work/primer question


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#1 grt222

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Posted 08 April 2013 - 08:01 PM

I've been getting my body work to the point where my primer fills in the fine scratches. I wet sand it and get it glass smooth. When i apply another coat of primer or top coat, the scratches or seams i filled reappear. It doesn't it with every brand primer i use. Am i trying to fill to much with primer? any help on this?



#2 Mooneyzs

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Posted 08 April 2013 - 08:14 PM

when you wet sand and block the body what grit of paper are you using to sand it?? I will sometimes use 600 but most of the time I will go 800 to 1000 when I block my bodies out. I also lay a guide coat of flat black misted over the primer when I block it so it will show me any low spots that I may have. As far as the scratches and seems you are saying reappear, have you put any sort of body filler in those areas. I Personally only use bondo and evercoat part #400 for my body fillers. They are both 2 part fillers where you have to mix with a hardner. Sometimes If I spray too much of a heavy coat it will make the filler pop and you can see it. So I usually mist a few light coats at first before laying down a little bit of a thicker coat. Then I will slowly build the primer up. I am using Plasti-Kote spot filler and primer that is lacquer based. Not sure if I have helped at all.



#3 grt222

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Posted 08 April 2013 - 08:32 PM

Thanks chris, i have been using 800 grit and bondo 2 part also. I use the plasti-kote t-235 or t-237 primer. I have thought about how i have been applying it and i put it on pretty heavy and not misting it. basically being in a hurry. I will have to try building the layers up see if that helps.  



#4 Ace-Garageguy

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Posted 09 April 2013 - 03:54 AM

Applying primer heavily and not letting it flash between coats, and not giving it enough time to shrink-in before final sanding and paint may be a cause of your problem. I often fill 180 grit scratches with primer, even though I know better, but allowing 10 to 30 minutes between thin coats, sanding smooth after about every four, and then allowing at least a couple of weeks for the last coats to shrink before final sanding seems to do the trick. Recoating primer without sufficient flash-time between coats traps solvents under a dry surface layer, and they have a difficult time getting out...but they WILL get out eventually, and shrinking, cracking and even flaking may be the end result.

 

Remember that non-catalyzed primer dries by evaporation of the solvents that give it a viscosity thin enough to allow spraying. As these solvents evaporate, the primer loses volume and shrinks, sinking into any sanding scratches in your filler. If you've sanded the primer surface flat before the shrinking is complete, the primer will continue to sink into the scratches under it (and will pull the paint with it) creating visible sanding scratches on the surface of the paint.

 

You might carefully check your filler mixing ratios too. Even slightly improperly mixed fillers can be overly sensitive to primer solvents, and what you identify as shrinking could be a slight swelling of your filler as the primer solvents leach into it, and then shrinking over time as they finally gas out. I learned many years ago that being in a hurry is a sure-fire way to have problems, and to be disappointed with my final product.