I have had success with the Island Girl line of gelcoat sealants. It's not cheap but it does work well. I did the two step process of applying the product called Elixir first and then a second stage called Mirror Hard. This was after buffing out the hull with a polisher and compound (which they say is not necessary; but was faster than rubbing out by hand with the Elixir product).
I applied this last spring. I hauled the boat in February and expected to need to polish out the hull again with the buffer. To my surprise, the hull was not in need of a major re-do. It had a chalky film of salt deposits and chalking gelcoat from my deck which has not been sealed or re-painted. Use of the Island Girl Pink cleaner diluted in water removed the chalking and did not remove any gelcoat. Once cleaned I re-applied the Elixir and it looks like new again. I should have just re-applied the Mirror Hard and been done. I'm waiting for warmer weather to do that job now that the boat is back in the water. All of their products wipe on and off easily. This product does not build up or yellow like Poli-Glow. Here is a link to their website:
Island Girl System - Cleaning and Restoration Products for your car, RV, Boat, Aircraft, shoes & more
I forgot to mention... The gelcoat on my boat is ~33 years old; and the IGP makes the boat look like it is newly painted.