Cold Weather Finishing

[Alex Carrera]

My basement shop gets too cold for finish this time of year. Right now it's about 60 degrees, and will probably stay that way for the winter. In my desperation to finish a small end table that I just finished, I've got the parts, I'm doing most of the finish before assembly, sitting under two 250 watt heat lamps. This seems to work so far with Rock Hard varnish for the top and General Arm-R-Seal for the remainder. However, am I weakening the finish or doing some other sort of damage that's going to cause me a headache down the road?

[Al Wasser]

I don't think 60 is too cold for finishing. I do a lot of finishing in the garage when the temp is in the 50s. Run some test pieces, but you should do ok without supplemental heat. The heat lamp may make it too hot. A small elect. heater should warm a small area if you want added heat. A little help with small projects is moving the finish and the project to a warm area and let them warm up and then move back to the cooler area to apply the finish.

[Jim Becker]

There are very few finishes that will cure effectively at low temperatures. 60º is generally below the recommended temp range for most finishes. Shellac is one...as an evaporative finish, it doesn't depend upon chemical curing to get hard. That may also be the case with solvent based lacquer, but I don't use it...and don't recommend anyone use it in an in-home located shop due to the dangerous fumes unless there is a formal spray booth with proper ventilation.

[Alex Carrera]

I love shellac, but knowing the abuse that this little table will probably suffer I thought that something a little more durable was necessary, hence the rock hard tabletop varnish. Lacquer is definitely out for me. I don't mess with that unless I can do it outdoors. Everything seems to be great for now. Both finishes are curing quicker than normal, but I've seen no signs of adverse effect, I'm just a little worried that I might be compromising their long term durability in some way that is beyond my current understanding of how this stuff works.

[Jim Becker]

Alex, to use the varnish, you'll want the temp to be at least close to 70º and the finish warmed up, too. Shellac is surprisingly durable...I sometimes chuckle when folks pan it the way they do...but you shouldn't use it if you are not comfortable with it! Note that if you use de-waxed shellac now, you can over coat it in warm weather with just about any finish you prefer to cover "long term durability". But shellac is also exquisitely renewable and repairable. Not so with varnish. The biggest challenge with shellac is that you cannot work it the same way you apply varnish...practice is important.

[Steve Schoene]

I don't think 60° is as bad with an oil based varnish as it would be for a waterborne finish. The waterborne finish just couldn't be applied evenly. But it will take a very long time to cure--I'd more than double the usual time between coats.

What I would do is give it enough time for a good part of the mineral spirits to evaporate and then bring it inside to temps closer to 70° to advance the curing.

I'd also consider some more powerful heaters for finishing time. If you use them just for a few days while finishing, it probably won't break the bank.

[Prashun Patel]

These guys are the xperts, but I'm not sure they've answered yr question re: whether heating the space will harm the finish in the long term.

I screwed up a shellacked piece by heating it excesively. I believe solvent was trapped under a surface that was hardening too quickly. Then, when it did escape, it blistered the surface.

I also screwed up a varnished test piece by putting on too thick a coat. I believe there again solvent was trapped under the surface. It hardened a little gummy and milky.

I'd feel comfortable doing what yr doing with the heater as long as yr not heating the piece directly.

[Jim Becker]

Shawn, you are correct to be concerned about heating the workpiece directly and excessively. What's most important is that the environment is warm enough and that the finish is also not "cold". The former means the room and I believe that's what Steve was referring to. The latter can be dealt with with a simple warm water bath if the finish has been sitting in a cold space. Pre-warming it means it will be within the proper temp range for application.

[Al Wasser]

Jim, I disagree. If we all wait for the 70s to get the finishing job done, it won't get done. I went out this morning. Temp gauge said 62. I applied the finish - Antique Oil. I checked a couple hours later and its dry to the touch. Sure it hasn't fully cured but I am not waiting til next spring to do my finishing. I'll wait til tomorrow to do another coat.

[Jim Becker]

Al, the temperature recommendation is for "best results". As Steve stated, for an oil based varnish, you can put it on at a lower temp. It's just not ideal.