home *** CD-ROM | disk | FTP | other *** search
- Newsgroups: rec.models.scale
- Path: sparky!uunet!mcsun!news.funet.fi!funic!nntp.hut.fi!saha.hut.fi!c34657b
- From: c34657b@saha.hut.fi (Tuomas Viljanen)
- Subject: Paint drying mechanisms
- Message-ID: <1993Jan8.232420.18079@nntp.hut.fi>
- Sender: usenet@nntp.hut.fi (Usenet pseudouser id)
- Nntp-Posting-Host: saha.hut.fi
- Organization: The Nibelungenlied Incorporated
- References: <C0E5Bo.IGu@usenet.ucs.indiana.edu> <3096@tymix.Tymnet.COM>
- Date: Fri, 8 Jan 1993 23:24:20 GMT
- Lines: 57
-
- These has been discussion whether or not old paints can be used. It depends
- much of the paint type and in which conditions it has been stored.
-
- Paints can dry (form a plastic layer over the surface to be painted) in two
- ways: either physically or chemically.
-
- The former means that the binding agent (varnish) of the paint is a thermoplast
- and it is as a solution in the paint: when the solvent evaporates, the binding
- agent forms a thermoplast film. It can be dissolved again in the solvent, if
- desired. an example of this are the acrylics. No chemical reaction occurs.
-
- The latter means that there occurs a chemical reaction as the paint dries.
- A thermosetting plastic layer is formed, as the paint is applied. The
- film is in the paint in monomer state, either as a solution or as an emulsion
- (usually water-based), or it is in two components to be mixed immediately
- before applying (like epoxy resin based paints). When the solvent evaporates,
- the monomers undergo a reaction between each other and with the oxygen of the
- air (the saturation of unsaturated fatty acids in alkyd resins to form cross-
- linking between the monomer molecules). This reaction is _irreversible_ and
- once the paint has been cured, it cannot be removed by just washing it away
- with solvent. The paint has formed a thermosetting plastic, and, in theory,
- the whole surface is one huge molecule. Common enamels are an example of this.
-
- So what does this concern on old paint usability ? A great deal. If the paint
- is of physically drying kind, the situation is in control, just add solvent and
- stir _thoroughly_. The situation is more difficult if the paint is of chem-
- ically drying sort: it depends on whether or not the paint in the can has been
- exposed on the oxygen of air. If the paint has never been used, the can is
- factory sealed and/or nice liquid-like sound is heard when the can is shaken,
- there are good chances that the paint is usable. If the lid-can joint has not
- been airtight, the paint is likely to be unusable. If, after stirring (perhaps
- with a motor tool) the paint can be put in even, running state, it perhaps
- can be used, though with caution; if the leakage has been big, it is likely
- that the paint has gone into "irreversible gel" state. In this state the
- varnish has reacted into a net structure, in which the pigment and solvent
- is bound in a jelly-like state, it is unusable and to be thrown away. If the
- paint, after stirring still resembles an aladobe, throw it away; it has gone
- into irreversible gel. Neeedless to sayif the paint has dried thoroughly into
- a solid block, it can be used only as nose weight for a trike landing gear
- planes, and it is usually too light for even this use.
-
- If your paint is of chemically drying type, the best thing to do with their
- storage is to make sure that the cans are airtight. That is, to turn the
- threaded caps tightly or to pound the lids of tins in securely... well, you can
- seal them aby casting some molten candle in the concavity between the lid
- lip and the tin wall. Another thing you can do is to use plastic photo film
- containers as storing vessels; they are airtight and you can easily see the
- shade of the paint if they are translucent, like those of Fuji. But always
- remember to clean the edges of the lid and can before closing the lid and seal-
- ing it. If the lid level is at higher state than the tin edge level, there are
- good chances that the paint will be, or has been, wasted.
-
-
- --++ Tuomas Viljanen ++ For a battle like Crecy, you do ++
- ++ Lahderanta 20 A 19 ++ not need a military genius like ++
- ++ SF-02720 Espoo FINLAND ++ Edward III. All you need is an ++
- ++ 358-0-592175 or c34657b@saha.hut.fi ++ idiot like Duke of Alencon. ++
-