mold making----sanding problem

I am making a 23ft fishing boat mold, the mold surface must be very very shining and glossy, how to get it ?
Some people told me when making the male mold,after painted the gelcoat I should sand it to #2200 sand paper, some people said #1800sand paper is enough. what should I do ?

 

Compromise with #2000?

-Tom

 

You should get some advise from the supplier of the sandpaper and buffing / polishing materials. They should easily provide you with a paper, explaining the steps to get your gelcoat to a perfect shine.

I must admit that all systems are basicly the same, sanding from a course grit to a very fine grit, and at some point stop sanding and start compounding, then polishing.
At what point to stop sanding and start compounding? Depends on the materials, but around 2000 seems fine. It also depends on your compound.

There is no harm in sanding with too fine sandpaper, so go crazy on the mould, I would say...

 

If you want a really shiny mould you have to go right back to the begining to getting the gelcoat for the mould .
There are a few simple things that you need to be aware of before you start .
What makes gelcoat non shiney ? no matter how much you sand and how fine a paper you use it never gets that really deep shine like chrome .
Its the chemical reaction between the catalyst and the gel coat together it produces minute gas bubbles !! how do you get rid of those small pockets of gas ?? if you are spraying the gel you have to spray fine at low spra pressure so the gas is able to disapate out of the droplets as its sprayed .
So you have to spray fine and multi passes of the gun till you eventually get to the required thickness of gel before the first layer of glass gets applied .The right choice of catalyst for the gelcoat being used is critical plus low pressure spray plus a little thinning of the gel plus the temprature is best warm and also the gel coat need to be warmed a little .
The best moulds i have ever seen the gel coats were always brushed with a wide soft brush , The gel was applied and brushed 6 times then another dip of the brush then 6 more strokes of the brush and so on till the whole job is finished . The brushing squashs the gas bubbles out of the gel when you pass the brush over the gel each time . Theres a art to it and takes a while to learn the techneque .
When sanding a brand new mould you need to use fine paper straight away ! the deeper you sand the more bubbles you will cut into and thats what causes the dullness that will never shine .
I always use a small air orbital sander and water to snad with . The orbital wont leave sanding marks like hand sanding does sometimes !!

 

Harder percentage and harding time, and spray gun calibre size ?

I Still like to spray not brush. Plan is to spray male mold two times, total thickness is about 0.85mm, is that good ?
My friend told me the percentage of the harder is very important, the second layer should add some thinner is that right ?
How big of the calibre should I use ?

 

Hyboats -
Cook Composites publishes a small book known as the "Cook Book". It has been the bible of gel coat and mold making for many years. They explain how low spray pressure causes porosity. Gel coat atomizes at the gun tip @ 55 psi. Without that atomization, it just doesn't do what it's supposed to do. See if you can get a copy of that guide, it'll make your life a lot easier.

 

http://www.ccponline.com/products/products_cookbook.html

 

THE GOOD OLD COOK BOOK
Had a copy of that for many years !Lost the original , then got the up dated version .
You can call it a bible but like a bible its just a guide and lots of good stuff to make one think .
Its a guide and should be treated as such .
Products vary greatly from country to country so you have to adapt to what you have in front of you , but in saying that the problems that every one exsperiances world wide are the same and the same causes !!. Nothing changes !!
Last big set of moulds i was a part of making we used a pressure pot to spray with . Maintaining a constant temprature 24 hours before hand was critical ,also having all the product and equipment in the same area stored on a table up off the cold concrete floor .
It was winter , South Korea , minus -15c outside and 2 feet of snow on the ground !! We had a really big tent and a just as big heater to maintain +20c ,the humidity was way down to 10% and less at times .
With a pressure pot you can have total control of the spray and material flow supply to the gun . The air at the gun also has a lot of adjustments also the flow rate out of the nozzle, the fan width should also be adjustable .
The plug surface area needs to be measured accurately and then you know how much product has to be applied . Divide the area onto sections and then you know as you are spraying that you are putting on the correct amount per sq/mtr or very close to it .
One person to spray , one person to mix the gel and clean the mix equipment between each mix , one person to fetch and carry and keep an eye on hoses and everything else in general , Its a team effort with no distractions till the jobs done !!
Two fine mist sprays backed with the third and final spray to get the required thickness needed .
Thinking about applying a second coat is playing with danger of getting it wrong some where and leaves to much to chance .Personally i would not do it .
When i set up a pressure pot and gun i have 2 regulator valves one for the pot and one for the gun supply . This makes the whole system fool proof and very versatile for other uses .
A really good water separator and line filter is vital when using any spray equipment , also check the airline connections and retighten all the clips holding the hoses to the fittings .Theres nothing like having a airline come off and flail about banging a wipping about all over the place .
There will be a pressure setting that suits the product you are using on the day, there always is !!! .
You can read pages and pages and i could write pages but its up to the person doing the job to get the feel of whats going on to do a good job .!!