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#1 Pavel

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Posted 28 April 2006 - 05:56 PM

I read before all the time that its okay to leave your car in primer (cellulose in this case) since it's non porous, so won't let any moisture through, it's only filler that is porous. but in another post pikey said that leaving a car in primer will attract moisture hence rust...

Anyone have a definite answer for this?

#2 pikey7

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Posted 28 April 2006 - 08:31 PM

It depends on the type of primer. My attitude though is to treat it all as hydroscopic to be on the safe side!

The point I was trying to get at, is that primer does not do anything to protect the paint from rust, and if anything it's porous properties make it easier for rust to get it's necessary moisture. My comments were to a guy who wanted tips on prep, whom I had no idea on his timescale before painting. It was my intention to make sure that when he was doing this prep, he wasn't doing it to be painted 6 months down the line when the primer had effectively caused no barrier between the metal and the outside world, had actually retained a bit of moisture on the surface to keep those dreaded tin worms well fed, and what looked to be a nicely prepped car had trapped moisture in under the primer coat, and would only mean a re-prepping and respray would be needed at a later date. To leave it a week or two won't make a real difference.

Obviously it's not disastrous as you can still wetsand most of the different types, and as long as it's dried before the top coat goes on, there's no real issue doing so. All these guys you see driving round in nicely primered cars waiting (or saving) for a spray job are just asking for trouble!

Cellulose primer is one of the least porous/hydroscopic primers, (I wouldn't go as far to say it's non-porous, but as near as damn it!), which is probably why you've read that its OK to leave it primered in that stuff (but then there's leaving it primered in the garage, and theres taking it for a 400 mile drive on salty wet british roads.....). Whereas 2k primer (usually polyurethane) is one of the worst offenders, and I think it's actually reccomended not to even wetsand 2k primer! Personally, I wouldn't even trust cellulose primer to protect sheet steel from moisture longterm.....

#3 anger management

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Posted 29 April 2006 - 07:44 AM

I read before all the time that its okay to leave your car in primer (cellulose in this case) since it's non porous, so won't let any moisture through, it's only filler that is porous. but in another post pikey said that leaving a car in primer will attract moisture hence rust...

Anyone have a definite answer for this?


I'll clear this up 4u!!!!

Primer IS VERY POROUS, as it acts like a glue for the paint to adhere to the panel, it does not (like some people think) protect the panel from the elements.
My advise would be to coat the primer, even with a cheap can of aerosol or something if there is a big time delay between primer and finish coat, until you get round to spraying in the colour you want, as your most likely to sand/flat the panels down anyhow for the respray!
Good luck

#4 Pavel

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Posted 29 April 2006 - 10:10 AM

Cheers guys, Ill have the car in cellulose primer no longer than 3-4 weeks, and its like 30 degrees and totally dry round here now, so it should be OK. Was never gonna leave it in primer forever, just was curious about the general consensus of the whole primer thing.

#5 Jae

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Posted 23 May 2006 - 07:05 PM

hmm.. if this is the case then isnt wet sanding primer the worst thing you can do?

#6 jack_marshall

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Posted 27 May 2006 - 11:47 PM

I wouldnt be wet sanding primer. Asking for the fresh paint to be going dull rather quickly or micro blistering.

We dry flat primer. I dont see why you would want to wet sand it anyway? Its flats off very easily, especially the high build stuff or polyesther filler primer.

If your priming a whole car, its a good idea to let it stand a couple days before putting top coat on. Filler and primer do sink and if you fill, prime and paint things in a day, they sink and look terrible not long after.




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