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On Mon, 11 Aug 2003 18:40:58 +0000 (UTC), "ken"
Can any-one enlighten me on the mastics used on the corner joints of a
motorhome and the best way of removing the excess where it has squidged out?
I haven't used mastic on caravans but I've used mastic (silicone
rubber) on leak-proofing pressure gauges for years.
There are some important things to know about it. They come in both
acid-cure and neutral-cure types. You can recognize the acid-cure
(acetic acid) types by their strong odour of vinegar. Acid-cure types
generally provide a more secure bond, but they do tend to corrode some
metals while setting (unless you can provide plentiful ventilation to
prevent the vapours from lingering).
Preparation of surfaces prior to application of mastic is important,
because the mastic clings best to very clean, somewhat porous
surfaces. Also, its not good enough to merely overlay the surface with
a bead of mastic. The mastic should be actively worked into both
mating surfaces. Take a pointer from the space shuttle programme. When
space shuttle Columbia was first carried aloft on the back of its 747
carrier aircraft, many of its insulating tiles fell off. The tiles had
simply been applied to the craft by pressing them into a filling of
mastic. Subsequent research demonstrated the need to "work" the
surfaces together, actively smearing the mastic into the pores of the
surfaces and keying a much better seal.
If you have an existing situation which is leaking, don't expect a
good repair by merely squirting mastic against the offending joint.
Water will find its way through it, sooner or later. You have to
either (a) lift the entire panel, clean the joining surfaces
thoroughly before mastic rejointing, or (b) design a form-fitting
flashing that fits the whole faulty joint, and mastic-seal that entire
flashing to the original surface.
Don't take too long over the application time. Especially in warm
conditions, mastic begins to set quite quickly. Get the stuff on
quickly, give it a good working into position, and then immediately
wipe off any excess with a rag. Complete the job in stages; only apply
as much in each area as you can complete within say 3 minutes.
While the mastic is setting, don't disturb or distort the area of
jointing, or you can work stress lines and air bubbles into it,
producing a joint that might leak. Give it at least 24 hours for a
Basically, nothing dissolves mastic once it has set (but then, that's
one of its big advantages). The only way to get excess mastic off
after it has set is to cut it off with a very sharp knife (and it
never looks as good as it should afterwards).
Mastic, when properly applied, is brilliant at leak-proofing, and can
absorb moderate vibration and shock without breaking the seal. I
started using silicone rubber compounds to seal front bezels and rear
covers of pressure gauges for liquid-filling them 24 years ago. Since
then, many of those gauges have returned for calibration to date, with
no leakage problems.