spraying polyurethane - a problem

I've been using an HVLP set-up to spray polyurethane onto restored/refinished furniture. One problem I've experienced is some mottling that occurs. The finish has a distinct "orange peel" finish in places. This seems to occur only when thick coats are applied, as the finish is always perfect after the first coat.
Perhaps I'm waiting too long (or not) between coats.
Another thing that's bothered me is that I am using lacquer thinner to reduce the poly. Perhaps acetone or MEK is a better transport vehicle.
Any feedback greatly appreciated.

 

Orange peel is usually cause by the material being sprayed being too thick. Thinning the poly a little more should fix that.

I wouldn't recomend using lacquer thinner to thin the poly. it wouold be better to use paint thinner [mineral spirits] If you need a hotter thinner, you might try naptha.

 

Ive found using acetone or MEK as thinners require a lot of skill. As the solvents in the overspray tends to dry in mid air before it even hits the ground or in this case some other part on your work peice. Causing a rough, gritty finish.

Jim

 

orange peel can be caused by many different things and sometimes what someone is calling orange peel can be other things. Typically orange peel from an HVLP can be common. HVLPs have a lot of air coming out the front of the gun but usually do not atomize as well as other types of guns. this can lead to a few issues. 1) the material is not atomized enough to flow together to create a nice smooth coat. 2) the material is dry spraying, i.e. the air dries the particles prior to them reaching the surface of the wood. 3) the air off the gun is drying the material on the surface of the substrate before it gets a chance to flow together. 4) there may not be enough material on the substrate to flow together. 5) the air flow in the shop may be such that is is causing the surface of the coating to dry before the base and tiny bubbles are forming. this could appear to be orange peel.

some things to look at before you spray. first use a viscosity cup to check the viscosity of your material. this should have come with your gun unless it was a very cheap gun. the viscosity the product should be at will be on the product information sheet or technical data sheet you can get from your supplier. if your product is in line with the range, you may wan to look at some sort of retarder for your product, if you contact your manufacture they should be able to get you the right product. if the viscosity is too think, then use the appropriate thinner (usually mineral spirits for polyurethane). lacquer thinner is a blend of solvents that may have no effect at all on your urethane, or they may inhibit your ability to achieve a nice finish. second look at your turbine or air compressor. if you are running your HVLP off of a turbine, the air will get hot, this could lead to problems 2 and 3. again use retarder. your air compressor, should you use one, may be pushing oil or water through the lines, either of these are incomputable with your product, your orange peel may be fish eyes. (I have heard people call fish eyes orange peel). third play around with your needle size. the manufacture of the HVLP usually does not know what needle will work best with your product. I have run into this time and again with my suppliers so now I don't even bother to check what they recommend, I just play with the needles until I find one that does what I want it to do.

the problem you are seeing on your second coat is probably in the first coat as well, it is just so small you cannot see it. you can try a microscope to see if this is the case. These are about $10-$15 at radio shack.

if all else fails try to see if a rep can come out and take a look. If any of my customers are having this issue, I go out to look however, I do not go out to look if the person does not and will not buy anything from me.

 

I believe my compressor is putting out clean air, but I will try a filter on it to be sure.
Based on my experience, again, the first coat goes on well....so I think the second coat is reacting with the first coat to produce the "dimpled" surface. Note: it doesn't occur on all surfaces....it looks like it occurs mostly on the top surface which gets the thickest coat.
I am most definitely going to try regular mineral spirits vs. the lacquer thinner. I have a suspicion that the lacquer thinner is "picking-up" the first coat and causing the problem...due to it's strength as a solvent. My other thought was to try a urethane reducer, which is mild as well.

 

to check your compressor take a clean white rag and blow air through it for about 10 minutes or so. long enough for the compressor to run and turn off again. You should not be able to tell you ran air through the rag. If the compressor is pushing oil, the rag will be discolored, and if it is pushing water then the rag will be wet.

a moisture trap at the compressor does not work, the air is too hot. put the trap as close to the gun as possible. Grainer sells a 5 micron filter that attaches to the gun, this works fairly well. Does your shop have a rigid air supply like copper or pvc or is it just a soft line coming form the compressor to the gun? if it is ridged then get a moisture/oil trap and put it at the end of your ridged line where the soft line connects.

 

Using mineral spirit will give the poly more time to flow out. If that's not slow enough you could try Penetrol by the Flood Co..

 

I am using a disposable filter at the gun. Thanks for the tip on the Penetrol as I have used their Flowtrol product with great success mixed with acrylic paints.
I am surprised no one has asked about the time-between-coats. I've been waiting about 10 minutes. Perhaps that is not enough for this slow drying material.

 

Poly and most house paints are quite different from automotive paints - you can't keep building up the film thickness. Usually you will spray 1 coat and let it dry, sand, dust and then apply another coat. You can spray 2 coats in 5 minutes or so of each other but it then needs to dry so you can sand before applying more poly.

FYI - while I'm fond of flood's product line I seldom add penatrol to anything I intend to spray. It slows down the drying time making it easier to get runs

 

Thanks much....and bingo, I think you've hit it on the head. I have sprayed acrylic urethane car paints and their clears with great success...but the prices today are unbelieveable. That's why I wanted to try the poly. But apparently the Poly's drying characteristics do not allow for multiple coats within 8 hours or more.