Replies

1. 
   SgianDubh | Jun 03, 2002 11:36pm | #1

   Sprayed vinyl sealers are used primarily where you perceive you might have a problem with initial coat adhesion-- perhaps an oily or resinous timber, or they are used for stain blocking, and most manufacturers claim superior moisture resistance. They can add a very slight milkiness to the clarity, and are often non-stearated formulations so unless you think you are likely to have one or more of the problems above, you can omit this layer and use the pre-cat itself as the sealer. In a kitchen I can see that you might perceive that you have at least one of these problems.

   As to sanding between coats of nitrocellulose based products such as pre-cat I seldom use anything coarser than 400 grit stearated paper, or 0000 wire wool with which knock off the nibs-- I prefer the paper. (You might use something a bit coarser over your first coat if the sealer raises the grain significantly, but if that's the case I'd tend to suggest you raise the grain first with a  cloth dampened with warm water and lightly resand prior to applying any polish.) Subsequent layers of these nitrocellulose finishes slightly remelt the lower layer so that they become an homogenous whole. The advice you've received sounds more like it relates to varnishes where in certain circumstances you need to provide a 'key' for the subsequent layer. Varnishes don't melt layer into layer like shellacs and nitro-cellulose type finishes, although I sometimes employ a wet-on-wet varnish application technique that reduces the need to sand between coats.

   Another good first coat under any film finish is a de-waxed shellac, and  Zinnser make a just such a product called Seal Coat designed for this purpose. I've been experimenting with this stuff recently to good effect on some restoration work where I was suspicious that existing polish may not have been quite as clean as I'd like. Slainte, RJ.

   RJFurniture
2. 
   Paul_Snyder | Jun 05, 2002 12:46am | #2

   Check with your coating supplier before doing anything. Ask them how many coats of sealer are okay (usually one is enough), what's the re-coat time (if two are okay), what grit to scuff sand between coats, and if the pre-cat you plan to use is compatible. Get the same info on the pre-cat - most have a recoat window and maximum dry film thickness. Some pre-cats should not be used over vinyl sealer.

   I use 3M fine sanding sponges, Klingspor 220 sponges, Klingspor 220 non-loading sandpaper, and Mirka Abralon 180 for sanding between coats of finish. The finish I use has a high solids content and the scratches left by the sanding do not show through the next coat. I have used some finishes that needed to be sanded to 320 so that the scratches would not show through the last coat. You coatings suppier will know the grit that is most appropriate for the vinyl and pre-cat you're going to use.

   Paul

   F'burg, VA

1. 
   SgianDubh | Jun 05, 2002 04:47am | #3

   Paul, you said, "Get the same info on the pre-cat - most have a recoat window and maximum dry film thickness."

   I'm curious about the recoat window you mention regarding pre-catalysed lacquer. I've not heard of this before. I ask because I've sprayed pre-cat one coat after the other, three coats maximum in about an hour or an hour and a half to achieve a complete finish of the recommended 5 mils maximum. I've also re-sprayed furniture I made years ago that came back to me for refurbishment, and  after some sanding back I've sprayed a new coat or two. Can you elaborate on the time window you mention and what circumstances might be lead to film finish failure as your warning implies? Most of the products of this type of finish that I use come from ML Campbell. Slainte, RJ. RJFurniture

1. 
   Paul_Snyder | Jun 06, 2002 12:36am | #4

   Sgian, If you ask your ML Campbell rep, they will provide you with specific re-coat directions for their Magnalac or Magnamax (or other product you may use). Magnalac has proven quite sensitive to improper application and will fail long before expected. There is a "window" of time in which you can re-coat pre-cat and get chemical bonding. After that time, you must allow proper cure time before scuff sanding and re-coating to get a mechanical bond.

   Different coatings have different spray schedules. Some pre-cats will cause the previous coat to buckle or blister if improperly applied. You haven't had any trouble because your spray schedule fits the material you're using very nicely. I pointed out the thickness issue because some finishers with less experience than you may not know how thick they apply each coat and what the total dry thickness will be once the finish has cured.

   Pre-cats are not the only coatings with re-coat time specifications and thickness limits. When planning to use any coating besides NC lacquer or shellac, it's a good idea to get the specifics from the distributor or manufacturer.

   Paul

   F'burg, VA

1. 
   SgianDubh | Jun 06, 2002 01:50am | #5

   Thank you Paul. I'll investigate further via the ML Campbell rep. when i get chance. I asked because I've never had a problem with their products caused by recoating windows, and one of their pieces of literature that I have states, and I quote, "ML Campbell products with the exception of Clearlite Plus do not have recoat windows." I'm well aware of the film thickness requirements though to prevent cracking. Learnt that the hard way, ha, ha, about a year after the client had been using the piece. Luckily it was some removable panels that I could strip and repolish easily enough.

   In the end though, I don't think any of the nitrocellulose based lacquers are my favourite finishes. They are quick to apply, look good, and can save a lot of time at fabrication, therefore cost to the client. They seem to me best suited to commercial jobs such as retail outlets, banks, etc., which are generally ripped out and replaced within fifteen years, so issues such as extreme long term durability, and subsequent repairs needing specialised equipment perhaps aren't the primary concern of the maker. Slainte, RJ.RJFurniture