Official Luthiers Forum!

Alan with this process what is you cure out time on the rockhard? Also what is the thinned viscosity or ratio of thinner per oz you are using? Are you using Behlen's rockhard thinner?

I add about 20% reducer by volume to the Rockhard varnish. Don't use hardware store paint thinner! When I cheaped out like that I got a white precipitate in the can, and it started taking longer and longer to dry. If you use the Behlen's reducer you won't lenghten the drying time to speak of. Do wait for the first coat to fully dry before doing the next one, though. Sometimes on oily woods it takes a bit longer, and if you rush it the secongd coat will just take forever to set up, and the film won't get as hard as it ought to.

Varnish that thin can be runny. The trick is to pull it on as thin as possible, and get it to thicken up as quickly as you can. I use a few drops of kerosene per pint of varnish as a brushing agent. It seems to rise to the top of the coaot and lubricate the brush a bit, so that you can pull it out more.

Adding about 5% of acetone also thins the varnish out, and since it flashes off fast it helps the coat thicken up before it has much time to run or sag. This also seems to help the new coats to 'bite' into what's on the surface, reducing the problem of 'witness lines'.

The main thing, though, is that if you have a really good surface the varnish will go on nice and level. Again, it seems as though the kerosene helps it to self-level a bit. I sand between coats with #2400 Micromesh, to get rid of the 'sand' and dust, and that is usually all the leveling that's required.

Be sure to put the last two coats on in the same day. This also helps reduce the witness line issue. I usually get by with about 5-7 coats after filling.

Allow the last coats to set up for a week or two before polishing up, if you can. This will also help reduce witness lines. I normally use #2400 and #3200 Micromesh, and then go to the Novus polish. I'm told that Meguire's 'Deluxing' compound really brings out the final shine, but I have not tried it yet.

Al, what's #2400 Micromesh equivalent to in US grit?
I use 400grit open coat / non load paper between the coats. I wonder if I could get away with finer grits and still get good mechanical adhesion.
I find that witness lines diseappear with buffing on the wheel to fine, and then super-fine Menzerna compounds. The Rockhard heats up pretty fast though, I am careful especially near the edges (don't even think of buffing the edges…).
I use hardware store turpentine and encountered no issue so far. The Rockhard is temperamental though, I adjust the varnish/turpentine/acetone mix according to heat and humidity, and how long the can has been open (it starts curing/thickening as soon as the lid pops out…). Seems to want to be brushed at 70º / 45% humidity (where it levels the best, for me at least).
I found out that the longer I wait to wet-sand the last coat, the better. As Al says, one week minimum. This varnish keeps hardening with time, I wonder when (if) it reaches equilibrium. I also wait a day minimum between final leveling and buffing.
The only issue I'm encountering sometimes is adhesion. It doesn't delaminate between coats but tends to lift, all coats at once, especially where raw wood meets finished wood, but also in random spots. I can't pinpoint the culprit, tried different fillers, schedules, sealers etc. On my next I will try Waterlox as a sealer, and we'll see if it improves something.
Anybody else with that problem (don't mean to hijack the post…)?

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/