Wood – How thick should should wood be to hold 160 to 180 pounds over a four-foot span

In my area, we have a place called Northern Hardwood Supply that stocks many types of woods used for furniture, custom trim, etc. I'm sure there are similar suppliers in your area. If your local lumber yard does not stock thicker woods, ask them if there is a specialty house in your area.

The second item worth mentioning, is that I recommend using a hardwood for you doors. A good choice would be poplar wood. It is the least expensive hardwood, easy to work with and finishes nicely, especially if you plan to prime and paint. Soft woods like pine tend to warp or wain and check easily and aren't the best for framing long narrow stiles etc.

As far as the tools and techniques for sizing custom stock, a table saw and planner are the best. The term "resawing" refers to cutting the thickness of narrower stock, say less than 4" with a table saw. One would set up using a feather board and a 80 tooth or hollow ground planner blade. This often gives a decent sandable cut, but not a real furniture grade finish. A planner is your best bet for a decent initial smooth finish that can be sanded to a good finish. The combination of using both is often required for custom sizing. Usually one side of your stock will have a good factory finish ready to sand, so do your DIY cutting all on one side of the stock that will not be as visible in the finished product.

Absolutely do not use an expanding gap filler, it will make matters worse. If you have verified that each step is level so that it doesn't need to be re-secured to the tread than the gaps can be filled. Also verify that the steps are not loose or shifting.

The easiest and most used product to fill each gap would be a latex caulking. A cartridge (or tube) is placed in a caulking gun and applied in a continuous bead along the gap. Once the gap is covered press it into the gap with a putty knife. This will also flatten the caulking so that it conforms with the flat wall. A very damp rag will wipe up any wayward caulk that didn't get pushed into the gap.

It will take about 24 hours for the caulk to fully cure. It than can be painted if desired, but there are plenty of colors to choose from which may not make painting needed.

As been suggested by a prominent authority well versed in caulking and fully supported by myself, you must be sure to use a Latex-type caulk. Not to be confused with a 100% silicone or a urethane caulking. Latex is easier and more forgiving than the other types.