Create a clean, contamination-free surface

Last week we covered Step 1 (surface preparation) of my troubleshooting “cheat sheet” to help you quickly identify and avoid most finishing problems.

(Thank you to Vern W., Lawrence M., Joe K., Michael A., and Bill K. for your blog comments and kind words.)

Today, we’ll move on to Step 2: Stain and Topcoat Compatibility.

Tips for Using Water-Based Clearcoats with Solvent-Base Stain or Topcoats

  1. Yes – All EMTECH brand water-based finishes can be applied over semi or fully cured solvent/oil-based stains and topcoats. I get this question regularly when a customer is switching to a water-based finish—and it’s the easiest to answer!
  1. If you are refinishing a piece that is finished with a solvent-based coating it is important to inspect the adhesion and film formation quality of the old finish that is being restored. If the original coating shows any type of adhesion failure or surface defects that cannot be sanded level, then the old finish will need to be mechanically or chemically stripped down to the substrate.
  1. If the old finish shows good adhesion to the substrate and there are no major defects visible (repairing defects is a completely different side-bar topic) then you can proceed with cleaning the surface to be refinished with a mild, alkaline cleaner to remove surface dirt, grime and contamination, followed with a careful but thorough wipe-down with a solution of water and denatured alcohol mixed 1:1. This wipe-down will remove any residue left by the initial cleaning process and will help to neutralize any free alkalinity that may be left on the surface.
  1. The next step is to lightly fine sand the existing finish with the right sandpaper grit schedule. I cannot stress enough that you DO NOT want to be too aggressive with the grit size of the sandpaper being used. Aggressive grits can leave deep scratches that can telegraph up through the new finish, so for surfaces that only need a light scuff sanding I recommend 400-grit aluminum oxide sandpaper as the ‘go to’ grit. Some grades of 320-grit are OK, but do not go any more aggressive than that. Water-based/waterborne coatings have very good adhesion qualities, so you are not sanding to create “tooth or bonding” scratch – you are sanding to level the surface of any defects that may telegraph up into the new finish.
  1. It is OK to apply water-based sealers or topcoats over a solvent/oil-based wood stain. You must keep in mind that many consumer and commercial grade solvent stains contain a percentage of mineral spirits – and this is the culprit that causes most of the failures between the WB finish and the solvent stain. It is critical that you follow the dry-time schedule as written by the stain manufacturer, which tends to be between 12-18 hours. Once the prescribed dry time has lapsed it is critical that you remove the oily residue that resides on the surface of the stain in question. That oil is mineral spirits, or parts-thereof – and if it is not removed it will cause a blush, haze or powdering effect when the water-based clear coat begins to dry on top of it. It can also cause poor film formation, i.e. fisheyes in the wet film formation of the WB as it is leveling. If you see this effect, you need to STOP and start over. So how do we beat this contamination problem – EASY – Wipe the dried solvent-stain with our go-to water and denatured alcohol blend (1:1) to remove any of the greasy diluent that is residing on the surface of the wood stain BEFORE you begin the sealer or clear coat application process. I think this part of the procedure is starting to sound familiar, yes?

    EMTECH USH3000 UltraSeal-WB Shellac Sealer

  1. The use of a dewaxed shellac sealer as a barrier coat is sometimes required. Substrates that are highly contaminated with oils or sealants — or are stained with a high-solvent content — should be treated with a light application of fresh, one-pound cut dewaxed shellac in the color grade of your choosing. A great option is our USH3000-UltraSeal WB Shellac Sealer and Barrier Coat, as it is made from natural dewaxed shellac flake and imparts a warm amber color tone in a water-reduced format. Yes, you can use Zinsser SealCoat® as a barrier coat, but ensure that you reduce it with alcohol, but it should not be used underneath any of our Emtech® pigmented lacquers or primers. We’ll be getting to surface preparation for a painted surface in our next installment.

As we talked about in Step 1, starting with a sound, clean, contamination-free surface is the first way to achieve stain and topcoat compatibility–and ensure a successful final finish. And when you follow the above tips, using water-based finishes over solvent-based stains can bring you beautiful, and durable, results.

I hope these simple tips help explain how your stain and topcoat can “play nice,” regardless if they are oil/solvent-based or water-based.

Keep an eye out for the next item on my list: “Step 3: Primer and Pigmented Topcoat Compatibility.”

Until then, do you have any stain/topcoat advice of your own? Or have any questions about stain and topcoat compatibility? Please share your thoughts or read what others are saying in the comments section below.