I just finished a model and I figured I would try a Tempera wash to bring out the panel lines. It worked fine on the top side of my plane, where it had a camo scheme, but on the bottom, where it was gray throughout, it left pale black marks along the borders of the lines that wouldn’t wipe away. I had used Model Master 2936 High Gloss clear coat, from the spray can, to cover my paintjob prior to decaling and another coat to seal the decals afterward. Then I laid down the Tempera over that coat, let it dry, and wiped it away. The same process was used for top and bottom, with the exception of the clear coats which were applied to both sides before the next step, and between the spraying of the last gloss coat and applying the Tempera I allowed about 24 hours for the clear coat dry. Did I rush the Tempera stage?
I’m also thinking about trying out Future on my next project, will I have to drop the Tempera for panel highlights?
All coatings, clear included, tend to be drawn into corners of any kind, leaving a thicker layer in and near those areas. This is a result of surface tension, and there is no way to prevent it.
As a result, the coating in and near panel lines is thicker than elsewhere. It takes longer to dry and longer to cure. What often happens is that after the surface is dried and cured, the coating below the surface still contains small amounts of solvent. This solvent migrates away from the panel line (where the coating is thickest) to the thinner areas at some distance from the panel edge. Where the solvent finally migrates out of the coating, the coating is softer and uncured for nearly as long as the heavier coating at the panel line. It can take up to twice as long for such areas to dry and cure compared to the middle of the panel. Applying your wash to this slightly sticky or possibly faintly rougher surface resulted in pigment being trapped there.
It sounds from your description that this is what has happened. This is a common problem in all paint applications, not just scale modeling, and it is worst when a heavy coat is applied, instead of multiple light coats.
You may be able to remove the stains by polishing the surface with 2000 or finer grit.
Essentially [#ditto] Mike’s post, with the caveat that high relative humidity and low temperatures extend both drying and curing times. If, for example, I was in St. Louis in July, without air conditioning, I’d wait more than 48 hours at ambient. Florida would be similar.
Drying and curing can be shortened by gentle warming, such as in a food dehydrator or similar apparatus.
With my drying booth I can gloss coat with Future and be ready to decal in an hour.
Then I decal, let it sit overnight and then gloss coat again with Future, one hour later I apply the wash, then another hour and I spray the clear flat. [tup]
Just find an appropriate sized cardboard box and line it with aluminum foil.
Cut a small hole in the top and attach a light socket into it through the top and screw a 100 watt bulb from the inside to sandwich it in place.
I put the model in there and turn on the light and put a towel or old T-shirt across the front to keep the heat in. I have a thermometer in it and it gets to about 110-115 degrees inside the box. About an hour in there and it’s dry.
The only time I had trouble removing tempera from a model was the first time when I did not glosscoat it prior. But I see that you did that, so the problem you describe should have not happened (!) Can you post a pic?
I can’t really post a pics, can’t find my camera, but what was said before I think is right. I just rushed it. Everything I’m working on now are kitbashes of fighters I designed for a book I’m writing and serve more for reference than display pieces, so I don’t mind experimenting and learning new techniques on them.
Mike, you can find small ones sometimes in gardening supply places. They are used for air circulation over sprouting plants. This is also a perfect place for a small muffin fan.
I personally would never put a 100 W bulb in contact with cardboard. What I would do if I didn’t have a food dehydrator would be to stand the bulb in the box opening with a small fan blowing past it.
It’s not the amount of heat that a 100W bulb produces—its surface temperature may be below the ignition threshold of cardboard. But cardboard is a remarkably good insulator—it transmits heat that it absorbs very slowly. So when you place it close to a good source of heat, it keeps getting hotter until a state of equilibrium is reached or it ignites.
The foil certainly helps, as it is an excellent conductor and reasonable reflector. Nevertheless, it is quite thin—I’d still put the bulb on the bottom.
Go ahead, call me paranoid. Accidents are things that should not happen, but do.
The voices in my head tell me that you aren’t being paranoid.
However, I have above my head is a CE, UL and ESA approved lighting fixture that uses a sheet of cardboard with a foiled side to seal the bulbs from the electrics. The bulbs come within 0.25" of the cardboard. It is designed for incandescent bulbs, and is an glass enclosed fixture designed for bedroom ceilings. It holds three bulbs rated for no more than 75 watts (225 total). If this is considered safe for use in residential bedrooms, Mike’s drying rig should be fine.
I don’t use a drying cabinet of any sort, as I work on many different projects simultaneously. I have a build table for gluing. A paint table for brush painting and detail work, and my spray booth. I just leave whatever I’ve painted in the booth for 24 or more hours. it has doors on it, and it almost completely dust free.
Bill, I’d be willing to bet that the cardboard in those fixtures has been treated with a fire-retardant chemical.
If I were to do it the way that Mike describes, I would unfold the box, lay it flat, and saturate the cardboard with a saturated solution of baking soda, a.k.a. sodium bicarbonate. After it was dry, I would then reassemble it for use. When sufficiently heated, bicarb breaks down and releases carbon dioxide. Won’t make it fireproof, but will improve its fire resistance.
I wonder if the voices in your head are the same ones as in mine? Maybe we’re on a party line? [:-,]
