If I remember correctly, T-111 is a reverse board and batton siding made from what is essentially plywood. This stuff shouldn't be left unfinished for any amount of time. We used to apply the primer to this stuff before it was even put on the house. If your siding is just bowing, you may not have much of a problem, other than the fact that your siding is bowed, and unless someone else here has any suggestions, I think the only way to fix that is to replace the siding. If your siding is de-laminating, you have a problem, and you'll need to replace the sections that are effected get a finish on them right away.
For a finish, I would start with Cabot's "Problem Solver" ALKYD Primer. I feel it's important to use an alkyd (or oil-based) primer on all exterior bare wood surfaces. Exterior oil primers can be really sticky, so I use K1 grade kerosene (A.K.A. "White Kerosene") as a paint conditioner. Do not condition the whole gallon at once as it is strong stuff and is easy to get too much. Use only what you need to make the primer flow on easier. Keep in mind that kerosene will extend your drying time quite a bit, if you get too much it will extend it redicululsy.
Next, if your siding is smooth, use paint, if it is textured, use stain. If you intend to opt for a semi-transparent stain, then skip the primer and just apply 2 coats of stain, otherwise use two coats of solid body stain or paint over the primer. Don't cheap out on materials! Nothing cost more than doing it over in a couple of years.
Some guidelines for selecting materials:
Oil materials stand up to air polution and acid rain better than latex, so if you live in a metropolitan area, use oil finishes. Latex materials stand up to the sun better than oils and are more resistant to mildew than oils also. So if you live in a rural setting, use latex finishes.
For finish materials I recomend using Benjamin Moore. In fact, find a small Mom and Pop Ben Moore dealer. You'll pay more but you tend to recieve better service and problem solving.