A water based topcoat. Anything oil based will provide a slight color change though perhaps not enough to notice it.

What do the instructions say from the finish you applied?

Where do you live and what are the climatic and humidity conditions during the year?

My guess is that you haven't used the proper wood or finish for an exterior application and it is subject to change due to weathering.

Exactly what materials and products were used here? That doesn’t appear to be exterior grade anything. That might need to get removed.

Depending on which part of Texas, blowing rain and humidity are likely common exterior weather conditions. Do the instructions for the stain you used say it's suitable for exterior applications? If not, you should look for an exterior clear sealing material with a long life span.

For exterior application, the wood should have been sealed on both sides. Because you didn't do this, humidity will cause these boards to swell and cup. You are going to find yourself doing this all over again in a year or two.

I would think it would be ok for the moment. I wouldn't panic about it. I would treat with an exterior oil or water base stain for protection when you get the chance. You won't have to put a seal coat on exterior stain. BTW, you can get in many colors and transparency's. But, probably easier to put proper exterior stain on now.

My parents have a outdoor deck, gets full sun half the day (in MN) was built before the pressure treated wood became popular, probably 1983 or so, is not pressure treated. They give it a coat of oil based stain every 2-3 years and is not bad, but should he replaced probably in the next couple years. It also was not stained on the underside, just top and sides of floor planks.

This was something to consider before the ceiling went up, BUT now that you've painted yourself into a corner (so to speak), look for a good spar urethane.

Thompsons will make a bad mix worse, visit a real paint store and follow their advice stat.

Waterseal and stain will do nothing for the termites, carpenter ants, carpenter bees, wood boring beetles, bacteria, and other wood munching pests. Protecting wood is not just about resistance to moisture. It's primarily about resisting insects and the microscopics that eat wood fibers. Moisture hastens decay, but it can happen even with dry wood and an attack of old wood borers or any of the many others.

CCA treated wood has been used since the 30's for exterior projects. A 1983 deck will assuredly be CCA. It was all CCA all until 2003 when alarmists insisted on eliminating it in favor of ACQ and others that are not as good.

This is why Azek and other plastics have increased in use tremendously.

I've seen Thompson's water seal flaking off outdoor surfaces. Don't know why, maybe in properly application. Just a out door stain alone would be enough. Wood fences with just stain do well.

Wood fencing is either cedar or another rot resistant wood or they too are pressure treated. The use of stain on decking, frnces, and other exterior woods is largely is cosmetic. It does add some water resistance. Thompson’s is a joke.

Because of where it is I'd leave it if a lot of the value to you is the way it looks. Get some of the same exact stain to keep in reserve for future recoating.

If there is no sun and rain and snow hitting the surface, I have seen wood ceilings that are stained and varnished last for 20 years and still look new. I don't think you have much to worry about here. Don't use Thompsons. I would use the Minwax water based spar urethane or the Last N Last Marine and Door spar varnish (water based).

http://lastnlast.com/Waterborne-Marine&DoorSparVarnish.php

I hate to say it but it looks exactly like mold to me. Even if treated with a conditioner before staining, pine doesn't take stain well but this wood has spots like mold spores. Make some test pieces and try different kinds of solid stains and paints on it.

The advantage of an exterior stain would have been mildew resistant additives and a finish coating would not have been needed.

Varathane Classic Stain (if that is what this is) is a urethane modified alkyd resin with inorganic pigments in a mineral spirits solvent. I suspect you'll need to use an alkyd primer first.

Find a store other than Home Depot that sells this stain and ask what kind of exterior finish can be applied over it. Or call Varathane.

Skip Minwax if you decide on a clear coating. Check out General Finishes. Choosing matte could give you something close to what you like. Do test samples. This one mimics the look of wax.

Can you please write an update, I have a similar idea for porch ceiling.