Patching and Plastering - grout, caulk, or both?

I have a tub that is starting to lose some grout along the edge between the tub and the bottom row of tiles. I plan on cleaning out anything that is loose with a wire brush. Then I suppose I need to re-fill with grout and then a layer of caulk over that. Is there any drawback to just filling in with caulk and forgetting about the grout?

plaese don't use a wire brush. your local hardware store will carry a grout remover. a wire brush will just ruin your tile.adding grout will protect water from getting under your tile and ruining your underlayment. silicone works in the seem way, but to do it right. first replace your grout, then silicone.

You need to remove all the grout and debris (old caulk) from the tile joint all around the tub line. Use a grout saw (hardware store) to remove the grout. After all the old grout is removed, clean out with a brush (not a wire brush, an old tooth brush will work fine) and vacuum out thoroughly. Remove all old caulk residue from the surface of the tile and tub.You can scape this off with a single-edge razor blade or window glass scraper (the new silicone will not adhere to previous caulk, especially old silicone sealant). You may want to consider filling the tub at least 1/2 way with water to add additional weight prior to re-caulking. This is done because when the tub is filled with water, additional stress is placed on the tub/tile joint which in turn will stress the sealant in that joint...sometimes to the point of breaking the seal with the wall tile line or at the tub line...it breaks the seal because it was made when the joint was filled with silicone when you re-caulked, and the tub was empty at the time. All of a sudden you fill the tub with water for a bath, and now its heavier, and pulls on the caulk joint at the wall tile and the tub rim. It sounds awkward to caulk with a tub full of water, but it's the best way to avoid joint failure.

The concept to understand is that grout is a cement-based product...it very rigid, and will not tolerate any movement. The reason the grout cracked in the first place is because the tub moved ( because of adding water weight or sagging, etc.,, and the grout remained rigid. Grout should NEVER be placed between two dissimilar building materials, with different expansion and contraction rates, as is the case with the tub/tile joint. So, the bottom line is DO Not replace grout in this joint...fill it COMPLETELY with tub and tile -grade silicone sealant only. You must use tub/tile silicone sealant only, as that type has additives that prevent mold and mildew from forming in the joint as water hits it regularly. You should place the top edge of the caulk line a little on the face of the tile above the tub, as well as a little onto the tub rim itself to provide a complete seal between the face of the tile and the rim of the tub. If you maintain this seal, the water will drain off the tile and into the tub, and not travel behind the tile and destroy the wallboard or plaster/framing. Please remember that the joint MUST be dry and very clean BEFORE replacing with new caulk..the sealant will not stick to a wet or dirty surface on the tub rim or tile face.