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After owning the boat for 3 years, it is time for me to revarnish the mahogany on my Offshore. As usual, I have a few questions ... !. Is it always necessary to sand down to bare wood? 2. If not, how can you tell? 3. Is it possible to get a decent finish working outside (my boat will not fit in my garage)? 4. How many coats? 5. Light sanding between coats? What grit? 6. Sponge or brush?

The varnish on my boat is beginning to show a few "cracks" - what I would call 'weather checking'. No flaking off or worn through spots. Is it a matter of sanding down until these flaws disappear?

Thanks for the help.

It is not nessecary to sand down to bare wood. If the finish has light weather checking you can usually sand down with a fine sandpaper and revarnish. As to the number of coats that depends on how deep a finish you want. The more coats the deeper the finish will look.
Best way to judge is by what you are looking for as far as depth.
Remember these boat when originally out of the factory did not have the fine finish you see most of us strive for.
Give is as many coats as you feel will make the finsih you want.
Depending on the type of varnish you are using will determine weather or not you will need to sand between coats. Read the labels.
I recommend a good brush for varnishing. Badger is the best you can get and I would recommend them. They are not cheap but they will give you the best finish. When it comes to spending money on a good brush the more you spend the better your final product will be weather it be varnish or paint. My Dad always said "Don't try to save money on the tools you buy becaus eif you buy a cheap tool you will be buy another one shortly there after". So do try to cut coners when purchasing your brushes.
Good Luck!

I use cheap throw away foam brushes for varnishhing. No clean up - just chuck 'em in the trash. And I have had excellent results.

No, you do not need to strip tpo bare wood unless the varnish is really bad. number of coats depends upon your condition and how long you want to spend doing the work. If you remove gardware etc... I'd say do three coats minimum, since you have taken the time to do all the prep work. Sure you can do this outside, but do not expect a perfect finish. You will get schmutz from the wind blowing around embedded in your varnish. Varnish is like a magnet for dust.

Remember to strain your varnish and never stick your brush into the varnish can - use a secondary container when applying the varnish and do not put any left over varnish back into the can.

Andreas