What Grit Before Finishing?

[Carlo Urmy]

Hi all,

I'm working on a project in maple and cherry, and plan to really go for broke on a smooth, flat oil finish. My standard technique is to use 3-4 coats of a combination oil/varnish finish (Watco, or similar) combined with wetsanding during one or more of the coats, potentially using up to a 1200 grit. I'm wondering how fine a grit I should work up to prior to adding my first coat of finish? 400-600 grit, or should I work my way up to a very high grit instead? Additionally, does it make sense to work through successive grits when wetsanding, or is just one round enough? If it matters, this project was machined on a CNC router, so I'm a little concerned that machining marks may come out during the finishing, even if I can't see them on the bare wood.

Thanks in advance.

[Steve Schoene]

I don't believe that there is any point in going finer than 400 grit for an oil finish. If properly applied there is no film to be polished. I would check for machine marks by wetting down with mineral spirits before finishing. You may need to be starting with something like 120 grit to remove tool marks. Then it is faster to work through the grits, though the end results just depend on the last grit used, as long as you make sure to remove scratches Stromboli coarser grits. I should also mention that for best results hand sanding. With the grain is needed for the final grit or grits.

[Carlo Urmy]

Thanks for the response Steve,

A quick follow-up: I have now finished the piece with five coats of watco danish oil. Either due to my technique, the rag I used, or the environment the last coat dried in, the finish, while looking beautiful, is not smooth to the touch and has a texture in certain spots. I suspect this comes from either finish seeping out of the grain, or from lint or dust getting into the finish as it dries.

I've found that I can smooth these spots out using very light pressure from some 1500-grit wet/dry sandpaper, but I have to be very careful to dust off the surface and use light pressure or else the sandpaper gums up and starts to mess up the finish. Is this a sign that my last coat hasn't cured enough, or is this always going to be a problem? For what it's worth, it's been about four days since I applied the last coat, and I left at least 48 hours between all previous coats.

Also, any advice on how to get a silky-smooth final coat without sanding after? I just couldn't seem to get that right on any of the coats.

[Scott Holmes]

You are leaving too much finish on the surface. After you sand-in the oil/varnish blend you should wipe it dry after about 5 minutes.

I would suggest you sand in the finish with a wetdry paper 400, is a good choice. Then wipe it dry with a soft cloth and let it cure over night. Repeat untill you like the look and feel. You can't build up a film finish with an oil/varnish blend; it's an IN-THE-WOOD finish only. It's way too soft and rubbery when cured to leave as a film finish. It still takes 30 days to fully cure, but it will not get hard... regardless of what the manufactures' "word smiths" tell you on the label.

If you would like to prove this to yourself, leave thin coat of the oil/varnish blend on a hard non-pourus surface and let it dry for a few days. Rubbery is the term I would use to discribe it.

[dan sherman]

I do this type of finish a lot, and I use the following procedure.

1. dry sand up to 200-240 depending on what grits your sandpaper manufacture of choice produces.
2. wet sand your first coat of finish using 400 grit wet dry paper, wiping off all excess & sanding residue. Depending on what oil/varnish blend I use, I might thin it out some.
3. Wipe on all remaining coats. I use & love the heavy duty shop paper towels for this. if you are wiping off the excess properly & soon enough you don't have to worry about dust nibs or finish build up.
4. after the finish has completely cured, I apply high quality wax.

[Howard Acheson]

When intending to use an oil/varnish mixture finish like Watco, sand your project up to 220 grit sanding the last grit by hand in the direction of the grain. Nothing is gained sanding to a higher grit. Apply your first coat of finish, keep it wet for 15-20 minutes and then wipe it dry. Let it dry overnight and apply the next coat. If you want a somewhat smoother surface, apply the second coat using 4/0 steel wool or 400 grit Wet & Dry paper. This creates a slurry of sanding dust that will slightly fill the grain. Lightly wipe off the excess. Once the item has fully dried for 3-4 weeks you can apply a coat of paste wax using 4/0 steel wool and buff. You will now have the smoothest finish that an oil/varnish mixture finish can create.

Remember, an oil/varnish mixture finish is intended to replicate an old fashioned "oil" finish. It's going to leave the "look and feel" of that type of finish. It's not a hard, glossy finish like a varnish. If you want that type of finish, it would be best to use a varnish or lacquer which is an on-the-wood finish rather than an oil in-the-wood finish.

If you are concerned about imperfections in the surface, wipe the item with mineral spirits as the final step in surface preparation. The MS will highlight any surface miscues so you can go back and re-sand.

Finally, you would be better off to make your own oil/varnish mixture rather than using Watco or some other pre-mixed product. Equal parts of your favoring varnish, boiled linseed oil and mineral spirits will create an excellent and more durable finish than Watco and produce the same "look" as the Watco.