Windows and portholes - that is where you will find rust also in the future. If you just want to stop the leak, there is a variety of marine
sealants/mastics/glues you can use. If you remove your windows, you will most likely find rust. For that, I can only recommend grinder and sandblasting - I believe that for a job well done there is no substitute for them.
On our boat, the window glass has holes drilled about 7” apart all around the edge. There is a sealant/mass between the window and the hull
and a stainless steel frame on top of the window. The window is secured on place with bolts that will go through the frame, window and hull
. The bolt is not the same quality of metal as the hull is, and no matter how carefully they are set after painting, they always seem to get in contact. That where the rusting on windows starts.
If you have never sandblasted before, do not hesitate. It is a very simple job, and for small areas fairly low cost equipment
will do. If there is a thick layer of rust, use a grinder to remove the most, then sand blast. That will save a lot of time. Just cover the nearby areas so that they will not get dirty and rusty. For small areas, I use a pistol with silicone head/tip. You push the head/tip firmly to the spot. The pistol has its own small sand container. While in operation, it circulates the sand from the container, to the tip and back eating the rust on the surface. There is very little sand flying around. I paid about $20 for this pistol (new) and it has worked now for four years. Always paint
immediately and always work on dry and warm weather
The methods suggested by Boracay will work as well. Especially, if it really is just a spot, just use Dremell or something similar. If it is anything more, go for grinder and sandblast. Acid is very good for deep cavities hard to access otherwise. Use whatever combination needed or preferred, but do get rid of every tiny spot of rust before painting. That way you are more unlikely having to do it again within the next few years - or on the worst case - next year.
Disclaimer: I am not a pro, I have just had a steel boat for some while. If there is a pro giving you another advice - go for it.