sploo said:
The MDF of my Nathans kept shrinking weeks after the last coat was applied. If you want to get it perfect you have to use very fast drying sealer (not water based like I did) or wait a very long time between coats.
Earl, are you referring to that stuff?
http://www.zinsser.com/product_detail.asp?productid=10
Best, Markus
markus76 said:
Markus
Thats the right stuff.
Matt
Thats what you should get and you should also take head from Markus. Don't rush the finish or it won't come out right - especially in the winter. the longer you wait between coats the better. The shellac is good because it does dry fast - very fast.
gedlee said:
Ehhh, it has Methanol as a denaturation agent so it's not exactly pure vitamins.
Use a real resperator like this:
http://www.coopersafety.com/item/100930/3M-Organic-Vapor-P95-Respirator-Kit.aspx
Shellac dries fast if sprayed in thin coats. If it's overbuild and lamps up, it may take a few days to cure properly, especially in a cold. Upper surface dries fast and traps solvents inside. Take your time. Everything cures slower in cold weather.
And careful with electric heaters and spraying near it.
Best, Roman.
R-Carpenter said:
Thanks for the correction, it just smells to me like alcohol. I never actually read the MDS.
But its also not Isocynacrolate (SP?) or whatever than thinner is in the two part polys - or MEK!!, that stuff will strip your nose hairs clean out, except that you'd probably be dead first so it wouldn't matter.
gedlee said:
Interested to hear about the results - though I found that lines would appear on the edges of the MDF (where one sheet is glued to another) only after several days, usually weeks.
I have used another shellac based product, but never got round to trying the Zinser. I do have some of it around, so it might be worth a test. One potential issue is compatibility with your paint coat. I have found some products are susceptible to attack, so it's always worth testing first.
Markus, the mask that R-Carpenter has recommended is highly advisable. Even non-isocyanate based products can be pretty nasty on the body - my advice is always to consider what value you place on your lungs.
sploo said:
I would suspect this to be due more to the glue than the paint. If the glue, or the parts, change dimensionally, even a small amount, then this kind of crack will occur no matter what kind of paint or primer you use. In this regard I have found epoxy to work best. But, with any glue one should wait as long as possible with the enclosure in a stable environment, before priming as all glues take a longer time to fully cure, and settle into their final dimensions, than to simply hold.
well an additional issue I have noticed is that glue joints are rarely ever perfect. Even if everything was done as perfectly as possible, mdf doesn't machine that well, and so you can have very slight, but noticeable depressions around the joint. Glue is also harder than mdf, as well as more dimensionally stable (right?), so as the wood changes with the weather, the glue doesn't. In the case of Dr. Geddes speaker, the glue joint is in the middle of the round over around the sides. My experience has been that it's important to do a lot of prepping before sealing, along with a lot of prepping after sealing. This means using some sort of fill on all seams before sealing, and after sealing. In the past I had used wood fill, as it's consistency can be changed as needed. However for this particular project I took Dr. Geddes advice and have been using bondo and surfacing putty.
I've been filling all the crevices with a thin layer of bondo, sanding that off, and then going over all these seams and crevices with surfacing putty, and sanding that off. This ensures that all joints are as smooth as possible before sealing. Then after sealing, I will go over them one more time to, again, make sure they are as perfect as possible. I find this almost always ensures that the lines don't show up again.
I think that, unfortunately the only thing I can do today is sanding. I was hoping to spray a coat of sealer on the outside, but its well below 0 today, and I just can't heat my garage up to a reasonable temperature without using a propane heater (probably not safe around spray equipment filled with alcohol).
I suppose I could always brush it on, maybe do one coat with a brush for now, sand that smooth, and spray the final layer for smoothness and consistency?
I've been filling all the crevices with a thin layer of bondo, sanding that off, and then going over all these seams and crevices with surfacing putty, and sanding that off. This ensures that all joints are as smooth as possible before sealing. Then after sealing, I will go over them one more time to, again, make sure they are as perfect as possible. I find this almost always ensures that the lines don't show up again.
I think that, unfortunately the only thing I can do today is sanding. I was hoping to spray a coat of sealer on the outside, but its well below 0 today, and I just can't heat my garage up to a reasonable temperature without using a propane heater (probably not safe around spray equipment filled with alcohol).
I suppose I could always brush it on, maybe do one coat with a brush for now, sand that smooth, and spray the final layer for smoothness and consistency?
pjpoes said:
I almost always do it that way. Setting up to spray when you know the surface isn't that smooth yet is just not worth it.
I have not had any trouble with my joints showing cracks, but it has been a lot of coats of paint to get there. I'm trying to get the number of coats down to as low as possible as this is a very slow process that is very time consuming.
all alcohol we consume is ethanol based. The fermentation of sugars causes the production of ethanol. Methanol however is produced in wood, and not sugar. Methanol is used as a denaturing agent, and while added to shellac, is not the primary solvent, thats typically ethanol. Methanol is added to avoid having to pay liquor taxes, and to keep idiots from drinking shellac.
