Problems spraying high build primer

I am spraying high build primer over epoxy. The primer is not coming out of the gun consistently. The sprayer is a cheap old suction feed unit. Sometimes the paint comes out when I squeeze the trigger sometimes not. I tried adjusting the amount that it opens with trigger pull, didn't seem to help. The primer is also drying sometimes before hitting surface. I adjusted pressure down to just above where it would pull the best. I disassembled and found no blockage. A friend who paints a lot more than I do has suggested a top feed unit. I have to pick up more paint so I plan to get one to try.

I have sprayed with a HVLP gun for PPG Omni 182. I use a 1.8 tip. It sprays fine, leaves a rough surface, but it has to be sanded anyway. I know nothing about suction feed guns, but if the gun is clean you need to thin it. MP 182 works fine without thinning, but gets slightly thinned with a ratio of 5:1 primer to hardener. With HVLP use the maximum pressure at the gun, which gives 10 psi at the tip. I have never messed with the pressure and the wide open pattern for topcoat or primer.

What specific high build primer are you using? It sounds like you may be using a lacquer-based primer/surfacer. Are you thinning it as per the manufacturer's instructions? Most lacquer-based surfacers that I have used in the past are thinned 1:1 (100%), usually with lacquer thinner. What tip size are you using? The older non-hvlp suction feed guns mostly use a 1.8mm (0.070") tip for general purpose use. That would be the minimum that you'd want to use for a surfacer product. A 2.0 or 2.2 might be more appropriate for the product you are using. Do you have a tech sheet for it? For non-hvlp, try 40-50 psig pressure measured AT THE GUN with the trigger pulled. The fact that you are using a suction feed gun is not your problem. They worked fine in the past before gravity feed became the "norm". Just make sure you are following written manufacturer's instructions, both for the gun and for the exact paint product you are using.

Also make sure the vent hole in your suction feed cup cover is not partially plugged. Using suction feed guns out-of-position with a very fast dry primer or surfacer, can sometimes tend to plug that hole. It also goes without saying that the gun should be disassembled and carefully cleaned after each use. So make sure it is sqeaky clean. Check for any strainer that might be partially clogged on the bottom of the gun pickup tube. If you strain your paint before putting it in the cup, as you should be doing anyway, you don't need that strainer at all.

I am using Nason 421-17 urethane primer with 483-87 activator. I don't know the size of the tip, it is the one that came with it. I checked the vent, it was OK. I found one of the four very small holes in the nozzle was plugged. The label instructions don't call for any thinning.

<a href="http://www.pc.dupont.com/dpc/en/US/html/visitor/common/pdfs/b/product/nsn/Nason/421-17_421-19.pdf">Here's</a> the full tech sheet on the 421-17. You probably have the general purpose 1.8mm tip that was pretty much strandard on the older gravity feed guns so that should be OK. Make sure your gun pressure (35-45 psig) is set AT THE GUN. I see the pot life is only an hour, similar to the PPG surfacer that I use. Don't let it sit around mixed before using. Spray it as soon as you mix it. As the pot life approaches and is exceeded, the stuff thickens up rather fast. That in itself could lead to spray problems with any gun.

(quoted from post at 11:52:52 02/13/12) &lt;a href="http://www.pc.dupont.com/dpc/en/US/html/visitor/common/pdfs/b/product/nsn/Nason/421-17_421-19.pdf"&gt;Here's&lt;/a&gt; the full tech sheet on the 421-17. You probably have the general purpose 1.8mm tip that was pretty much strandard on the older gravity feed guns so that should be OK. Make sure your gun pressure (35-45 psig) is set AT THE GUN. I see the pot life is only an hour, similar to the PPG surfacer that I use. Don't let it sit around mixed before using. Spray it as soon as you mix it. As the pot life approaches and is exceeded, the stuff thickens up rather fast. That in itself could lead to spray problems with any gun.

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Thanks for the information neighbor!

It sounds to me like the primer wasn't stired enough. Sometimes primers with a lot of solids takes a lot of agatation. If you pour it through a fine strainer you should be able to tell if there are any undisolved lumps. Sometimes a sprayer may appear ok and you take it completely apart and throughly clean it and it works fine afterwards. I also had one that developed a leak in the siphon tube where it screws on the sprayer. It's like drinking a soft drink through a straw with a pin hole. Not all of it gets there. I ended up having to coat the tube and threads with red coat fuel tank liner to seal it. Of course it can't be taken apart again but it was a cheap gun.