Roughness and Final Sanding with Waterbased Topcoats

      Roughness on horizontal surfaces could be overspray, but it could also relate to grain raise and sanding methods. May 13, 2013

I've been working on trying out a product that is new to me, Sherwin Williams Kem Aqua Plus, a water based clear sealer/topcoat. I really like this product and have only one issue that I haven't resolved. All my horizontal surfaces look good but feel a little rough and they are not silky smooth like the vertical surfaces are.

I'm currently doing these in a makeshift spray booth as my new booth isn't finished yet, so I'm suspecting it's an overspray issue as my ventilation isn't very good in the makeshift booth, or maybe itís dry airborne spray embedding in the wet finish. Does this sound likely? Is there a good way to confirm this? As I mentioned the vertical surfaces are perfect and I'm wondering if this will go away once the new booth is operational or if I have a technique issue I need to address.

Forum Responses
(Finishing Forum)
From contributor O:
I used Kem Aqua exclusively as a topcoat before the economy tanked in 2008-09 and I closed my shop. I can tell you that the spray environment was critical to the performance and quality of the finish. Unlike solvent based finishes, you have to keep in mind that you are putting water on wood and you will have some grain raise issues. Overspray can be an issue as the water base does not "burn in" like a solvent, so you can really feel overspray on the horizontal surfaces. Also, I found that I still used the Kem Aqua sanding sealer before topcoat with a 180 scuff sand in between the sealer and first topcoat. This allowed me to get a smooth build, even on open grain woods.

From the original questioner:
Am I correct in assuming that the grain raising is limited to only the first coat? The second coat doesn't re-raise the grain right? I did notice that the grain raising was fairly significant with the sealer coat but it was expected and not an issue.

I did speak with the local rep. We're up in Canada and have less access to a variety of help, but they are attentive and good to deal with. One thing that he wasn't able to clear up for sure is whether or not a scuffing is needed between topcoats. The best conclusion we came to is that after the sealer coat, scuffing is not needed if re-spraying within 30 minutes to 12 hours, and outside that window it should probably be sanded prior to the next topcoat. Would you agree with this approach?

From contributor O:
The grain raise is pretty much limited to the first sealer coat and depending on wood and the amount of grain raise we sometimes had to increase the coarseness of the abrasive to get a smooth surface to begin topcoating. We scuff sanded between all coats and found that to give a better result.

It was a bit of a learning curve as the dry times for the water based are much different than the solvent based and depending on how quickly you move parts through your spray area, into drying, and then back into spray for re-coat, then your results will vary. That is why we went for a two-three hour cure with a scuff sand in between coats. We found that with an average residential kitchen that was about the "cycle" time given the spray and drying areas, equipment and personnel (one spraying, one sanding, both moving product).

We also found that controlling the humidity was crucial. We were in the southeast where humidity is normally 80% or better during the summers, so we air conditioned the spray area and added dehumidifiers to keep the humidity between 60-70%. Too dry and you get flash curing on the surface. Too humid and you can't get the water to evaporate out of the finish to dry it.

The other thing that they may or may not have told you is that you can add butyl cellosolve to the Kem Aqua to enhance the flow and burn-in properties. We did this as well, adding a quart to every five gallon pail before we started spraying. This helped us tremendously.

From the original questioner:
Thanks for passing on the tip about the butyl cellosolve, he never mentioned that. I think that might really help with the larger pieces. I am up in the pacific northwest rainforest and humidity can be an issue we address on a regular basis. Knowing that the Kem Aqua Plus is sensitive to this I'll keep an eye on this as a variable in the shop when spraying. Thanks again for your experience with this product. The new booth should be done in a few days and I'll be able to run some more samples in a better controlled environment.

From Gene Wengert, forum technical advisor:
Try your final sanding using a fresh piece of sandpaper. Fresh paper with sharp particles will tend to cut fibers off rather than push them down. Hence, you will get much less grain raising. Light pressure is also preferred to get good cutting of the fibers. You can use a sanding sealer to stiffen the fibers so they will cut better, but this is an extra step. A closed coat paper will be better for final sanding than an open coat.

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