hi ya Martin
With regard to your question about using Epoxies and Polyester resins; to get the true answer for yourself, i.e. as to which one is the best; you will basically have to work with them both over a period of time.
They both have advantages over one another; and of course disadvantages; these are my thoughts on it;
Polyester ResinsThese require a stable temperature to work in, as you have noticed, say round about 20 degrees Centigrade, but no lower than about 15 degrees Centigrade. The hardener to resin mix ratio is pretty critical.
Compared between the two, pricewise, it is the cheaper one. For what we are going to use this polyester resin for, e.g. modelling, it fills all our needs quite easily. We do not keep our models in the water for great lengths of time (as in weeks on end :) ) So we need not really worry regarding water being absorbed into the polyester, because, in normal circumstances there is a protective coating over them e.g. varnish.
Before polyester resins start to absorb water they have to be submerged for at least 2-3 days.
With regard polyesters shrinking and moving; all plastics do this, even Epoxies. Some at a greater rate than others.
You have got to think about your application of the Polyester; you are not planning on building a great thickness of matting and resin up on your hull, e.g. something over 5mm. So, therefore, I would not be too concerned about it moving and cracking, because, do not forget, any form of timber as well would expand and contract as well under working conditions.
Where problems surface is if we coat one side of the timber with matting and resin and leave the opposite side open to the environments. This is because, if the timber side becomes damp, it will expand at a far greater rate and therefore delaminate from the polyester resin. So, that is why it is always adviseable to give the inside of the hull a coating of resin as well as the outside. This actually seals the timber.
The only major problem occurs when you build up a mass of fibre glass e.g. I have seen around bottoms of hulls and also rudder posts where people have laminated and poured resin in, to something say one inch thick, they think they are adding strength and this is where the polyester starts to deteriorate, move and crack.
Epoxy ResinsThere are several grades of epoxies. Several names too. No doubt you will have heard of what is called 'The West System' which was the first system really developed for Marine Use, but that is the 1:1 scale Marine Use. It is one of the highest water resistant epoxies and also one of the most expensive. It's mix ratio is also critical - from the top of my head I think it is 3-1 mix ratio, i.e. 3 parts epoxy to one part hardener. Sometimes to maintain this mix ratio, you have to purchase the correct dispensers.
Working with this - it is much the same as Polyester resin - its like a thin treacle. It is not so temperature critical either, some specialised epoxies can be used in temperatures just above zero. But, these are specialised epoxies. It does have a less shrinkage to it, for arguments sake, let us say one inch square stick of polyester resin would expand 1/8 of an inch at 50 degrees. One square inch of epoxy resin would only expand about 1/32 of an inch at 50 degrees. These figures are not exact, but, just to give you some idea. One of the major drawbacks is its price. I think it is twice the price of a polyester resin.
Also, it is a slightly more hard resin, it withstands impacts and indentations at lot better than polyesters. How do I know this,
the concrete at the side of the lake is very HARD a lot harder than polyesters and epoxies put together
.
I suppose last but not least at all
there are health warnings that go with them which I believe we have mentioned elsewhere.
So, to be honest with you, its another case of,
the choice is yours, if you are prepared to pay that little extra for the Epoxies, it will give you a longer lasting hull. If your hull is going to endure all different environments.
However, if you are just going to use your model say a couple of times a week on a standard lake, but from the pictures I have seen of some of the lovely lakes you have in Australia, we would class them as an Ocean
- the choice again, is yours.
Hope this is of some help to shed a little light.
(Please be aware though, I have just skimmed lightly over this topic).
Aye
John E
bluebird
Author
Topic: Fibreglassing a hull (Read 23972 times)