Depends on the varnish you use.  Some types say drying time above 70 degrees is six hours before sanding.  Others say 12 hours.  I've found that with most drying overnight at around 70 is plenty and you can sand without fouling 220 grit paper by morning and apply another coat. But, best to follow the directions on the varnish you are using.  I've only used 220 grit sandpaper between each varnish coat and have come out with a very fine/deep finish with four coats.  Good luck.

I don't have a lot of suggestions for varnishing, since I've never bothered to varnish my boat.  I might say, cut the varnish a bit and it will go on more smoothly and dry faster, based on woodworking experience. 

As for outdoor storage, I built a pretty simple rack out of 2  8 foot long 4x4s, 2 8 foot long 2x4s  some lag bolts and a 6 foot section of 1x10.  Just cut 2 J cradles out of the 1x10, and use the 2x4s as a top bracket and short legs (forward, below the boat) and you've got a pretty decent rack for ~$20.  Add a 3rd 2x4 to prevent racking (lateral movement) and it's just about perfect for about $25.

FrankP 

Forgot to mention, you can also attach a tarp for sun protection by just attaching it to the horizontal cross-beam at the top of the rack, and you'll be pretty satisfied with the protection you'll get.  I will also suggest a cockpit cover as I've had several birds nests built in my kayaks when they were stored outside without covers.

FrankP 

You can build your coats faster by not using such coarse paper.  220 between coats is for varnish salesmen.  400 wet is plenty of tooth for between coats.  You can even scuff it up with fake steel wool nylon pads - get the fine or extra fine, I like 3M pads.  All you need to do is knock the gloss and dust bunnies off.  Try wet sanding every other coat and using the pad for the others.  If the shop is warm enough you should get a coat a day easy.  220 dry on varnish can leave scratches that may show in the right light later on.  Fresh varnish is pretty tender and easy to sand so be gentle.  

Several of us was having some beers down Orlando at the 2008 Varnish Salesmens' Convention, and the subject of your post came up.  The fellows felt it was a cheap shot, and I was elected to give the official position of the Society to set the record straight.

I've got two in college, a number of us have variable mortgages that have gone up recently, except for Dave, he's got a twin-engine  plane that although he paid cash for it, you have your insurance payments and what not.

Anyway, we all feel that 220 grit is essential for a good bond, except Dave, who has recommended 30 grit to his customers for years and never had a complaint, at least not about tooth, although some of them do feel that Dave's suggestion of replacing all unused cans of varnish with new every 60 days is overly cautious.

So Mr. Miller, we have to make a living like everyone else and think that the whole  Varnish Salesmen Profession has gotten a bum rap, except for Dave who said he's retiring at the end of the year and doesn't give a rat because he'll be 28 and he's not getting any younger and he'd like to spend more time with the family and what not.  That would be Bridgette in his case as they do not have kids although you would have to wonder why if you saw Bridgette but she is by no means typical of the ladies of the Varnish Salemens Ladies Auxiliary.

Thank you for your attention.

And to think you did not invite me to your convention!  They association must have me blackballed since that time at the Coconut Grove ......

Actually varnish will stick to fresh varnish just fine.  The subject of "hot coating" or adding coats with no sanding at all has been on this forum as well as the Wooden Boat forum several times over the years.  Timing is critical but it does work.  Please note - if you try it and have problems then it must be bad varnish and not my advice!

 Next convention ask the can makers why they make such cheap varnish cans.  They never seem to seal right again after the first opening.  I have started using quart thinner cans with screw on lids that you can buy at some paint stores to keep my varnish instead of the can.