Hull thickness and seacock backing plates
As the weather is getting cold to work with sealants outside, installing the thru-hulls will have to wait until the spring. If we'll have a cold spring, I might make a tarp around the hull to be able to heat it up, but we'll see. Until then the interior can be easily heated and working inside with epoxy is possible, so I'm planning to install backing plates. The hull of the Scanmar 35 seems to be quite thick, I measured it to be 14mm (solid GRP laminate + the gelcoat), but some extra thickness might still be good to help distributing the load.
I consider this as a "once in a lifetime" job and since there is time till the spring, I want to do it properly, so hopefully I'll never have to do it again. Some people use plywood for backing plates, but I was concerned that if anything goes wrong and water gets in it will rot. So I followed the advice of this article instead and got some solid G10 GRP sheets in 3mm and 10mm thickness from eBay. The plan was to use the 10mm sheets for the larger seacocks and 2x3mm for the smaller ones.
To cut them to the right size is a challenge. These plates are so strong that only metal holesaws can cut them. The wood saws will just burn the sheet. Even with metal saws it nearly burned down my Bosch driller to cut through the 10mm, so I suggest working with thinner sheets and epoxy them together if you need more thickness. The 3mm sheet was quite easy to cut. To have a perfectly centered cut, it's a good idea to start with the big holesaw first but don't cut it through completely, then use the same center hole with the smaller holesaw, then switch back to the bigger one and finish drilling out the last few millimeters you left. Also don't start with the largest plate as I did (I know, it was stupid) because if you mess it up, you will waste more material. Now I will have to order some more sheets to properly finish this job.