Based on my woodworking knowledge, you'd have to sand off the Satin finish (down to the gloss finish beneath).  Satin varnish has particles in it that dull the look (unlike clear gloss that has nothing in the mixture).  Once the Satin is put on, you've applied a layer of particles that would have to be removed in order to get back to the clear sheen you're looking for.

I don't think it's the end of the world, but it may take (yet again) another sanding or two to reach a point where you can get back to the clear varnish, re-coat and bring up that original gloss.

Good luck,

Larry

I don't think you have to sand it all off.  Pick a spot to experiment on, sand it with 220 so it's well scuffed, and wipe it with a wet (water) cloth.  What you see while it's wet is what you'll get with the gloss varnish on top.

JackD,

Your theory will work, provided he sands through the two satin layers.  All varnish/poly begins as gloss; additives are used to "cloud" the finish, taking away a percentage of clarity (semi-gloss and satin have varying amounts to produce the desired effect). 

But yes, no need to sand through the original 4 layers of original gloss; just take off the particles that are clouding the issue, so to speak.

Hope this helps,

Larry

Absolutely unneccessary to sand the satin coats. Gloss or lack of it is a function of surface smoothness and nothing else. The particles put in the varnish are there to cause irregularites on the surface. You absolutely can't see the materials suspended IN the varnish. This is why fine sanding produces a satin finish. You haven't done anything the material underneath and yet the finish is satin. You can lay down ten coats of satin, prep sand it and lay down one coat of gloss and you will have a glossy looking boat. If anyone doubts it, try it for yourself and see.

I'll look forward to seeing the final report on that.

L

It's an easy test, Larry; try it on some scrap stock.  You'll be surprised.

Here is an article by Michael Dresdner (a well acknowledged finishing "guru") you may find informative:

http://www.woodworkersjournal.com/Main/Articles/Rubbing_Out_The_Final_Step_to_a_Great_Finish_5261.aspx

Here is an excerpt that I think pertains to this discussion ... "Finishes typically start out gloss, and are made semi-gloss, satin or matte with the addition of something called flatting agent, which makes the finish itself less shiny.

Flatting agent consists of tiny particles embedded in the finish, and they actually make the coating slightly cloudy. Look into a can of satin finish and a can of gloss and the difference in clarity will be immediately obvious. Apply enough coats of satin finish you will see a slight loss of clarity in the build. For that reason, it is often wiser to build up a thick finishes in gloss, which has better clarity. You can then switch to satin for the last couple of coats, if that's the sheen you prefer, or simply rub the gloss down to satin.

Going the other direction, trying to rub a satin finish to gloss, is not a good idea. While the surface will get a bit more reflective, the embedded flatting agent will prevent you from creating the sort of clear, gelid gloss you most likely want. However, you can rub satin up to semi-gloss, matte up to satin, or rub any finish to a lower sheen. "

 "Here is an article by Michael Dresdner (a well acknowledged finishing "guru") you may find informative:"

 I think this is where I was going originally.  But hey, I'm just a 3-decade old wood-worker, what do I know about finishes?

I hope the end result is what you're looking for,

Larry

The OP was speaking of a gloss finish or lack of. This has nothing to do with any purported lack of clarity in the substrates. Consider paint for a moment. The undercoats of paint have no clarity at all. Paint is of course opaque due to all the solids in it. And yet paint can range from matte to high gloss. The amount of gloss is a function of light scattering on the surface and nothing more. The smoother the surface, the less the scattering and the higher the gloss.  The presence of the flatting agent creates a satin finish by causing tiny irregularities in the surface to scatter light and not by clouding the substrates. Flatting agents are usually silica and are virtually invisible in clear finishes.

Regardless of theory, try this out yourself. Take four pieces of scrap and label them A,B,C and D. Put five coats (typical kayak finish) of gloss on A & B and five coats of satin on C & D. Now prep sand all of them and put on a coat of satin on B & C and a coat of gloss on A & D. After they dry show them to an experienced woodworker and ask which ones are all gloss, all satin and which have two different finishes. Better yet, do a double blind study. They will not be able to tell which is which. I know because I didn't believe this myself until I tried it.

This was an interesting discussion.  Has anyone tried it?