I previously posted a dilemna about prepping a 72 Chevelle for final professional painting (recap: My first time doing a whole car and I don't have any spray equipment and don't want let panels rust as I slowly work my way around the car).

I received several replies on this post suggesting to me that priming with a sprayer requires a much lower level of expertise and lower end spray equipment than final painting.

So based on the advice I went ahead and bought a new gravity fed spray gun off ebay for $30, a gallon of OMNI LV polyester primer from autobodydepot.com for $60 (what they recommended) and a $25 3M half mask organic vapor respirator.

Because I don't plan to use the system for anything but priming I figured I could get away with a cheap compressor and basically end up with what a "professionally" primed car for about $200 total.

I then ran into 2 problems. First, according to the instructions with the sprayer, I need 43-50psi and 7.5cfm. But when I looked at compressors I'd have to spend about $300 on a compressor to get this flow rate!

Also, I heard a rumor the primer will only be good for a few days before it chalks up! So even if I solve the compressor problem I'll end up having to sand the whole car and re-prime anyway!!!

So if this is all true then I'm back to wondering why I shouldn't just use rattle cans as I go along and then sand the whole car again and rent a sprayer to prime just before I get the car painted (or maybe let the pro prime it too)??

P.S. For the life of me it doesn't make any sense why the flow rates listed on compressors are lower as the psi goes up!!! Wouldn't more pressure drive our more liquid? Here's an example:
Delivery At 40 psi 4.9 SCFM
Delivery At 90 psi 3.5 SCFM

Gravity feed guns are air consumers. A cheap HVLP gun runs ~10-15 PSI.

Get a cheap HVLP gun.

I think what you mean by primer chalks up after a few days it the un-used liquid in your gun/can if left to dry. It takes weeks of fine sanding on primer to get a great paint job. The more time you spend, the better it gets.

Also the flow rates work that way for a reason hehe. When you fill a tank with air, its easier and faster when its empty and not much compression. But after you get some PSI built up, its harder to add air to the tank.

I try and watch my dollars spent on my hobby and my old Buffalo Brand spray gun broke last week after giving many years of service and I went to autozone and picked up one of these:

http://aircompressorsdirect.com/catalog/product_info.php?cPath=5_62_157&products_id=130

I shot some Omni epoxy primer with it and it worked so much better than the old gun that even though it broke it was probably past it's prime.

I'm running mine off of a Craftman compressor and what it rates at as far as output I don't know but it kept up just fine. It was maybe around $300.00 or so but I had it given to me by a family member.

As far as the chalking I wonder if it's a rumor. On my stuff I read the tech sheet on the Omni primer and after 3 days I have to scuff or sand it before shooting on another coat or anything else over it. Hopefully if the car sits while I do other area's on the car I can, right before I shoot it, sand the whole car down and then shoot the final color on it. The car will not be sitting outside so that should help.

Jim

I read someware that primer is porous so the final coat will soak in & stick to the primer. But being porous it absorbes moisture. If left in a moist place after priming it will rust from the bare metal out. Thats why good painters dread to paint over others primer jobs.

Larry, there are a few issues we are dealing with there. First off the "chalking" primer, BULL, total and complete BULL. Primers don't have UV protection (to speak of) and over many MANY months can "chalk" up and need to be sanded well before top coat. It would not need stripped or anything like that.

Second, why do YOU want to prime this car? There is no reason what so ever to just "prime" the car. It is not saving any money, it may not even be needed. The only real need to prime it would be to first, make a uniform substrate for the paint. Well, the painter can do that in the form of a "sealer" or "Primer/sealer" when he is painting the car. It is applied "wet on wet" where no sanding is required between them. So you priming now wouldn't be needed for that reason. Secondly, you would want to prime to "surface" or "Block" the car to a higher degree of perfection. Do you plan on doing that? If not, it is best left up to the painter which primer and the prep for it.

My next concern, and it is a big one, is the choice of polyester primer. Polyester primer is a VERY, VERY specialized product. It is for VERY, VERY high filling PERIOD. It is very thick, hard to sand, and should only be used where SERIOUS surfacing or blocking is needed. There is no reason what so ever to completly prime your average car with polyester primer. What are the reasons autobodydepot gave you to use this product?

Personally, if you have to go out to buy a compressor JUST to prime this car, I think it was bad advice to prime it yourself.

ASB, again, BULL. Lacquer primer, yes, but modern 2K primers are insoluable.

DG, let's clarify something here, the average HVLP gun needs 10-15 CFM, not PSI to operate. I am sure that is what you meant, I just want to clarify. smile.gif 10 PSI is not a problem, any cheapie compressor can do that. 10 CFM, that is a whole different story. Without the CFM, you just don't atomize the product and it is left on the surface full of solvents, not good.