Part 2
We left Part 1 with the gel coat in place in the moulds. The temperature dropped dramatically over the following days making progress slow and unsatisfactory. I did get a casting but the gel coat had failed in places. It can be repaired but I made a new casting.
Now is a good time to put on your PPE especially for your hands. When the gel coat becomes tacky you cut some fibreglass mat or cloth to completely cover the mould and carefully press into the gelcoat making sure there are no air pockets and that the mat is pushed into the corners. I use scissors to cut the mat so it fits.
Next mix some laminating resin with the correct amount of MEKPT hardener, details are on the resin bottle. Take time and ensure the mixture is mixed thoroughly. Using a brush cover the mat with the resin and stipple well into the mat. Keep working the resin into the mat until it is well wetted. Leave for at least 24hrs or longer until the surface is no longer soft and sticky.
I use a release agent that is water soluble and these casting fit in the kitchen sink in warm water. Use a thin plastic pick or piece of wood to separate the mat from the mould round the edges taking care not to mark or damage the mould. Take your time but the water will dissolve the release agent and release the casting. Any residual release agent will wash off the casting and from the mould.
The two halves can now be trimmed to size and put together and held with strong tape. Next step is to cover the joins on the inside of the castings with strips of glass mat or cloth. Mix another pot of resin, add a length of stick to your brush so it can reach the bottom of the casting and apply to the joints. I tend to be fairly liberal with this coat as it does need to be both strong and watertight.
When set remove the tape and sand the outside of the keel to shape round the joints. Make good any small imperfections with body filler and inspect the inside to ensure the resin and cloth covers all the joints, making any repairs as required.
I level up the open top by running through the bandsaw to remove any sharp edges.
The keel can then be painted to any colour required. If the keel has a damaged gel coat it can be filled with bodyfiller and sanded smooth to shape, bit of a pain but will work as well as one with a good gel coat.
Please note that this is a keel to hold lead underwater and the finish is not up to the standard of a hull where the two moulds would be clamped together and the gel coat applied to both halves resulting in a near perfect casting every time.

Part 1
As I am currently making some keels for my own use and another Model Boats Website member, I thought it might be useful to share some of the processes involved.
My friend Brian Clewes had previously made a mould to cast keels for his own yachts and has cast some for other club members and my models. He kindly let me borrow his mould shown in the pics with a wax and soap type release agents.

You will also need Gel coat resin, Laminating resin and MEKPT hardner. To clean up I use Acetone and Iso Propyl Alcohol, Paper towels and Vinyl Gloves. I use a small plastic bowl from a fruit jelly to mix and a small brush to apply.

The first job is to clean the mould with IPA then apply a thin coat of release agent and leave to dry (15mins in warm weather). When dry mix the required amount (marked on the tin) of Gel Coat and MEKPT hardener well to ensure it will all set. Brush the gel coat over the release agent making sure all is covered. The outer edges of this mould are not used so remain uncovered. Setting time will vary depending on the temperature but eventually the surface will go dry but remain tacky, ready for the Glass cloth and layup resin.
I will cover that in part 2