I have a ar lower with a gray, almost purple anodizing on it. I was thinking of using Brownells Aluma Hyde II in Flat Balck so it it matches the rest of the gun.
I saw the Videos on their web site. But I have a few questions.

Here's what I was planning. First a soak in mineral spirits to get grease and dirt out of the inside. The guy I bought it from ran a can.
second I was thinking a light scuff with a maroon scotch brite pad to give the paint something to grab.
Third a blast of Brakleen to clean off the scuff stuff.
and then spray and dry between coats with a heat gun set at low.
I think the temp in my cellar is mid 60's is that too cool for this stuff to dry?
How long after the last coat does this stuff take to fully cure?
Need help from people that have used this stuff before.

I'd go along with your soak first and scrub everything with an old toothbrush.  
Next, after wiping it dry I'd pop it in an oven (or use a heat gun) to "melt out" any remaining grease or solvent.
Then, give it a wash down with brake clean to rinse off what the heat "soaked out of it".  You'll be surprised what all the different roll pins and small parts can hold.  I don't mean a hot oven, maybe 200 degrees, just enough to get the buildup thinned out and moving. Brake cleaner has always left a slight residue on my stuff so I always use a final wash of denatured alcohol.  It evaporates fast and seems a bit better for the final.  So, use the brake cleaner, than scuff, than more brake cleaner and finish with denatured alcohol.

Alumahyde II is pretty thick and you have to apply it in thin coats.  It can be a little tough to avoid orange peel.  I've found the warmer - 80 degrees or so - the better.  I just read a can and it states to apply it between 70 and 90 degrees.  You can speed the drying process by circulating 90 degree air (MAX) over the part.  All subsequent coats must be applied within 30 minutes.  It cures to the touch in around 15 minutes but don't use (re-assemble) the part for 24 hours.  Also remember to warm the can itself, that helps.  This time of year is tough to get the best result unless you have a shop you can get up to 80 degrees.

Brownells says up to 3 weeks for full cure.

Since the lower is aluminum degreasing with hot soapy water is a very safe and effective method.  Just dry the receiver in an oven at 200 degrees for an hour.

I've used it many times. After completely cleaning and disassembling the part to be finished, I hit it with (residue free) brake cleaner and let it hang til dry. Then apply Alumihyde2 in thin layer. I haven't tried applying any sort of heat, but I know the instructions say you can. The trick is to let it cure long enough before you start putting things together again. It seems like when people have problems with this finish, it's usually because they get their paws on it too soon. It will cure at lower temps, just increase the time you let it sit. I typically let it cure for 3-4 weeks.

My only complaint is that the "dark gray" wasn't as dark as I would have liked it.. The matte and gloss black are quite good..

You really have to heat the part slightly before you start to spray. The paint evaporates really quickly when it hits the warm metal so you don't get runs or anything.

But like others have said, you want thin, thin layers. It can build up fast and then it takes forever to harden.

Shot it today. Soaked it in mineral spirits and then hosed it with Brakleen. I hit it with compressed air to dry it. Warmed the part with a heat gun and warmed it between coats. I skipped the scuff, the finish looked rough enough.
Looks good. I have it hanging down the cellar between the two furnaces and water heater. So it's about 55 to 60 degrees there. I guess I'll have to wait a month now.
A cheaper, easier alternative to anodizing.

Don't rush to reassemble. It can take a long time to fully cure. Especially if you sprayed it on thick. Don't ask how I know

Live and learn!

So I have 5 or 6 coats on it. How long should I leave it on the hanger? and how long to reassemble? The air temp where it's drying is 55 -60 degrees.

Easy way to get it to cure quicker is to put it in the oven at 250 degress for about 30 minutes.

What he said.  I just got done spraying the my stock,VFG,pistol grip along with 2 GI mags.  I put everything in the oven for a few hours and everything came out great.  Let it cool and put everything back together.

Without the oven trick 2-3 weeks. If you want to be sure it is 100% cured.
It may apear to cured to the touch but if you start to rough handle or assemble it you might run into problems. I know I did and it was in a 90 degree garage for a week before I assembled it. The Video even says " during the winter time it can take quite a long time for it to completely cure"

Your best bet is to follow the video on the Brownells websight and the stuff will make you happy!

Aluma Hyde video

Once cured it is a great product.

Just out of curiousity - why 5 or 6 coats?

Many spray finishes run easily and a run will spoil the entire project.  Multiple light coats are safer.