Gel Coat?

Brown parcel tape leaves its brownness all over your repair. I found this out the hard way. Insulation tape will do the job but isn't so sticky. Bog standard sellotape is fine because by the time the water washes it away the gelcoat should have set.

If you don't use too much gel coat and are careful how you apply the tape, very little sanding will be needed. Duct tape is OK but it too can leave a residue and becaue it is cloth based, may imprint the pattern of the cloth on the gel. Clear tape is a good idea as you can then be certain that there are no bubbles and you will get a smooth surface when you peel it back.

When you are sanding down, mask off the surrounding area so that you don't inadvertently sand down the "good" hull. Use the finest wet and dry you can get away with. 800 then 1600 then rubbing compound if you want it to look really good.

Three words of warning. Make sure you get the right hardener - there are a couple of types and they are not interchangeable. Wear disposable plastic gloves and your oldest clothes  cos gelcoat doesn't wash off - ever. Don't apply too much gelcoat - it will shrink very very slighly as it cures but gelcoat is very hard stuff and not at all easy to sand in quantity.

And don't expect perfection - at least not on your first go.