Of all the technical questions I field during my time speaking with customers, the number one inquiry I get from both professional and DIY customers alike is, “I am switching to your water-based EMTECH Series and I am not sure about surface preparation. Can any of your products be applied over a cured, solvent-based paint or finish?” As the use of water-based products continues to increase in restoration/repair applications and more people are switching to water-based finish, customers are down-right confused on how to approach the changeover. Here are a few important surface preparation tips that will ensure that your restoration/refinish project goes smoothly.
6 Tips for Switching to Water-Based Finish
- Yes – All EMTECH brand water-based finishes can be applied over semi- or fully cured solvent/oil-based stains, sealers and topcoats—pigmented paints included. That’s probably the first question I get when someone is switching to water-based finish—and it’s the easiest to answer!
- 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.
- 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.
- 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 then that. Water-based/waterborne coats have very good adhesion qualities, so you are not sanding to create “tooth or bonding” scratch – you are sanding to level the surface to to create a micro-scratch for the new finish to bite into.
- The use of a dewaxed shellac sealer as a barrier coat is sometimes required. Substrates that are prone to tannin bleed, or surfaces that are highly contaminated with oils or sealants, should be treated with a light application of fresh, one-pound cut dewaxed shellac in the color grade of your choosing. The use of clear shellac should be limited to wood species that will be finished natural/bright, or stained and then clear coated.
- Surfaces that are to receive a pigmented/painted finish should be treated in the same manner as above, but the barrier coat should be a pigmented shellac or acrylic primer that is rated as a “Stain Blocking” primer to prevent tannin stains from bleeding up into your color coats — **this is CRITICAL when finishing red or white oak**. Avoid using full-strength shellac-based primers unless you thin them with alcohol or water, 25 to 50%, to lower the solids content to prevent micro-cracking or checking of the top coat applications. Keep the primer coats thin, stay in the 1 to 2 mils per wet coat—avoid heavy primer coat applications.
Surface preparation and the rules of refinishing/restoration do not change much whether you are working with solvent or water-based finishes. Starting with a sound, clean, contamination-free surface is the key to a successful final finish–and when you follow the above tips, switching to water-based finish will bring you beautiful, and durable, results.