Pre-Cat Lacquer Seal Coat Sanding

Pre-cat lacquer works well as its own sanding sealer, but it takes some experience to learn to avoid bubbling in the coat over certain types of wood. July 18, 2008

I had a salesman out to my shop today that is trying to set me up with a different lacquer (precat) then I have been using. He brought a cup gun with him and was demonstrating the lacquer on red oak plywood scraps that I provided. I sanded the test pieces and blew them clean of dust. He used the precat lacquer as a seal coat.

The seal coat never really flowed out well and dried to sort of a bumpy texture. It also had some trouble with bubbles that got trapped in the lacquer. He then proceeded to spend a lot of time sanding out the defects in the sealer coat before shooting the top coat. When I saw him doing that I told him that I know it is possible to do some very careful and time consuming sanding and get a half way decent finish over a rugged seal coat but that I believed seal coat sanding should be a fairly rapid process if any money is to be made.

Is it normal to have to sand more painstakingly when precat lacquer is used as the sealer instead of a secondary sanding sealer? I really wanted to buy the product because it is supposed to be a higher solids and more water resistant. Also, it is quick drying stuff and did not clog the sand paper or ball up. He was shooting it unthinned but then thinned about 10% to try to solve the texture problem. It didn't really make much difference. Does anyone have any insight as to what the trouble could be? I am fairly sure my air supply is clean and dry with traps and filters.

I have a big job to shoot and it would be hell if the entire sealer coat had that sort of texture. I know from experience that it will affect anything else you shoot on top. My spray equipment is a pressure pot and Binks 2001 gun. I know there is better equipment to be had but I am stuck with what I have at the moment.

Forum Responses
(Finishing Forum)
From contributor A:
Vinyl sealer is the ticket - one coat and all you have to do is dust sand it. Then the precat lays on like glass. Now the bubbles are another thing. If oak is stable I don't get that, but if you have a weather change going on and the wood is trying to adjust to that weather, that's when I get bubbles. Take two pieces and shoot them both. Put one inside and the other in the sun and you will see what I mean. Bubbles are also worse if you shoot a heavy coat.

From contributor B:
The only finishes I will spray on 98% of what I build or spray for others are precat or post-cat. I can assure you that every one of them will bubble on open-grained stuff like oak if you spray it too heavy. Air gets trapped in the open pores quite easily. Add in some solvent pop, and viola, bubbles galore.

This problem is super-easy to solve. As mentioned above, vinyl sealer is a bit easier to manipulate and sand, but I don't bother with it. It isn't necessary. Every precat I've ever sprayed was "self sealing" and did not need a sanding sealer. Here is what I do with open-grained woods like oak/ash/hickory:

1) Seal it with precat. Skip the sanding sealer. It's just an additional product to keep up with, and honestly it isn't as rugged as precat. The trick to any good sealing is to thin by around 30% and spray it lightly. Just a wet coat is all you need. If the temps are hot you may want to use some retarder (get the kind of retarder the manufacturer suggests, some precats supposedly require different retarders).
2) Only thin the second and third coat enough for a good flow. Spray them as normal, around 3 wet mils.

They should all sand fairly easily. I have sanded entire huge kitchens with the same fine-grit sanding sponge so long as you aren't brutal on the sponge. As a side note, I highly recommend MLCampbell's MagnaMax. It is a fairly high solids precat, so keep some thinner handy.

From the original questioner:
I got a tip that the lacquer may have been too cold. It had been sitting in the open bed of a pickup truck and traveling for hours in upper forty degree weather prior to being brought into my shop. The shop and presumably the wood were @ about 65 to 67 degrees. The salesman had a small amount of lacquer in his cup gun for about three hours so I guess it takes longer to heat up if that was the problem. To contributor B: I appreciate what you said about the first coat being light. Like you, I find it more convenient and cost effective to use the precat for seal. When the seal coat is right, I realize it will feel rough to the touch, but what should it look like in diffuse light? I am guessing it should be glass flat?

From contributor B:
So long as you know it will be rough, yes, it should be flat. If you can see any orange peel or unevenness/heaviness (detectable by pronounced variability in sheen) then something is amiss. Either thin or warm the lacquer (or both), and spray lighter. I spray with a Binks 2001 every day Mon-Fri currently when I am spraying things for my employer (I can't talk them into anything more efficient) and that gun can flat hose out a ton of finish if you are using a pressure pot or quart pot.

Normally I use an HVLP gravity feed gun (the Porter Cable is only $100 and works well). It is much more user friendly if you ask me. It is also a bit slower, being HVLP, but plenty fast enough for lacquer.

From the original questioner:
To contributor B: I really appreciate the help. Since you are using the same setup as I am, would you tell me what pot pressure and fluid pressure work best for you? Also, a salesman is telling me that I will have no problem shooting a two part urethane product through my rig using the same fluid nozzle etc. What do you think?

From contributor B:
The old pot that they have for me to use right now has a really pathetic pressure gauge that I can't even read. I turn the dial until the needle is at around the 10 o'clock position. I have no idea what psi that is. Basically it's a lot more pressure than I'd normally need.

I over-guess the pressure and then dial back the amount of fluid coming out with the adjustment on the gun. Since I've sprayed such a hideous amount of stuff, all I need is a piece of cardboard to get the right mix of pressure and air.

We run our compressor at 120 PSI, which goes into the old pot (which is actually a Sharpe pot, not a Binks, only one regulator indicator). Our gun has a cheater valve directly connected to it where I dial the pressure in "by what sounds about right" and then test with the cardboard.

Before anyone criticizes this archaic setup, I have absolutely no say-so in the matter. If it was left up to me I'd replace it all with a good Kremlin, which I've used before and love. But, I do manage to get a decent transfer efficiency (based on the amount of fog I don't create, compared to what I see other guys do) and I also get a very nice looking finish. 2K Polyurethane? Sure. As long as the lines are nice and clean, which they should be anyhow. Be sure to flush the lines with the poly reducer before loading up the catalyzed poly. I'd also recommend you buy some empty gallon or quart cans to place inside your pressure pot. 2K poly has a 15-30 minute pot life, and begins to Jell-O on you very quickly after that. You'd rather pitch a disposable can than have to clean this stuff from your pot. Be careful to rinse your lines very well with lacquer thinner when you are done. If it hardens in your lines, you will be buying new lines.

From the original questioner:
Interesting what you say about the short pot life. The guy that is selling me this urethane says it has a four hour pot life. This is a Becker Acroma product, two parts. Anyone know about this stuff? It is supposed to tough enough for a wooden restaurant table top/bar top application.

From contributor B:
I have not sprayed that brand, so if they claim a 4-hour pot life, it probably is. They gain nothing by lying about it other than really angry customers. One big caveat about spraying things that will be abused like restaurant stuff nothing is bulletproof. I highly, highly recommend 2K Poly for such things, but I always remind them that there is not a wood finish on the planet that is unscratchable and foolproof. 2K poly gives you a nice, hard film finish that is nearly solvent-impervious. Chemical durability is the real flagship trait of 2K poly.

As a side note, do your best to talk them into a satin or dull finish. Most restaurants naively want a gloss finish, since it looks good for a few months. Just ask them if they have ever gone to an established, well run restaurant that had spotlessly shiny tables? Nope. I haven't. Dull or satin sheen will cover a LOT of scratches and dings.

From contributor C:
I'm a little miffed at some of contributor B's statements as well. Your fluid pressure going into your pot does depend on the viscosity of your lacquer and your fluid nozzle size, however 8lbs- 12lbs is a good starting point the 2001 gun is conventional so your atomizing air should be set at 35-40 lbs. What a lot of guys end up doing is ramping up the fluid pressure going into the pot and then choking down the fluid needle nut (don't do this). Adjust your flow rate at the pot with the needle nut backed all the way out.

From contributor B:
You say not to ramp up to the pot pressure to compensate at the gun, but you do not say why. Aside from the fact that it works like a charm for me, I honestly would like to know why not to do that.

From contributor C:

Most 2pk polyurethane products regardless of brand have much longer pot lives than 15 minutes. When you choke the needle in your asking for material to travel around the needle to exit the nozzle often your spray pattern can be inconsistent heavy top/light bottom vise versa and the needle will wear. Also you can create excessive overspray due to the velocity of your material exiting the gun.