Dear Vinny: Two recommendations:
(1) If you use wood again to replace the rotten end of the trim board, be sure to seal the open grain at the end of the board. That's why the original board (and many trim boards) rot. The open grain at the end is an invitation for water and rot. It takes only a few minutes and a little DAP 230 to seal the end grain, and block water infiltration. We do the same thing with stair stringers -- which have ~2x feet of open grain per length, created by the cuts for the steps and risers. Again, it takes only a few minutes to seal all of this open grain with DAP 230 or a similar sealant. (DAP 3.0 works well in temps below 55 degrees F.)
(2) If you need to replace more trim board, I highly recommend cellular PVC. It does not absorb water, never rots, never needs to be repainted, is easy to install and is not significantly more expensive than good wood trim. Plus, if you want a different trim color - cellular PVC holds paint much better than wood, because it does not absorb water. See the attached photo; we replaced all of the wood trim on our house along the shore with cellular PVC Here are some installation tips:
A few tips for installation:
(1) Always use stainless steel trim screws or trim nails to attach it, to avoid rust stains. The FastenMaster Cortex system is the best solution, with matching PVC caps that eliminate the need to fill screw holes. See: Cortex Model # FMCTXT-VA50SMHD, Home Depot Internet #203705494 Store SKU #822865
(2) If you need to join any corners, use heavy duty PVC glue (from the plumbling dept) plus an 18 or 23 gauge brad/pin nail to hold the corner together while the glue dries. Home Depot carries stainless 18 brad nails and 23 gaugue pin nails. You will also need a matching nail gun, e.g. the Ryobi One 18 gauge brad nailer, Model # P320, Home Depot Internet #203810823, UPC Code # 033287139620 Store SKU #632013
(3) If you need to add new window trim, build it as a 'frame' on the ground, then mount the frame on the window as one piece. This will allow you to work on a flat surface and build a nice trim box with tight joints. Pocket screws are helpful for large pieces, and glue all of the corner joints with clear PVC glue. For pocket screws you will need to buy a jig and use stainless screws. See: Kreg Model # K4MS, Home Depot Internet #202711579 Store SKU #1001213548 and Kreg Model # SDK-C2SS-700, Home Depot Internet #202711494 If you need a shorter screw, standard "pan head" stainless steel screws will work with the Kreg system. We prefer 'star drive' over 'square drive'.
(4) Corner boards and special shapes: Many people do not know that Home Depot carries a much wider array of building products -- not simply what is displayed on shelves in a store. This is important for cellular PVC projects. If you need long corner boards for new siding, for example, you can buy pre-made AZEK corner boards (exterior and interior corners) with 'rabbeted' edges for new siding. This saves a ton of time and money, and the quality is excellent. If you need corner boards or another special shape or size, talk to the Pro Desk at your local Home Depot -- and check the AZEK website for the options before you go. An image of an AZEK corner board profile with rabbeted edges is attached below.
(5) If you cut cellular PVC and the cut edge will be exposed, you need to seal it -- to avoid dust collecting in the tiny pores which turns gray or green. Sand the edge smooth with 300, then seal it with a clean rag that is damp with acetone. Wear gloves because acetone is absorbed directly through the skin -- and is carcinogenic.
(6) If you need to install siding next to your trim, you can create or order 'rabbeted' edges -- a deep groove along the edge of the trim that the siding boards slide into, covering the ends. We cut rabbets into our window trim with a table saw, and ordered rabbeted Azek corner boards through Home Depot. The photo above shows what a rabbeted edge looks like.
(7) You can mix and match HD Veranda and Azek trim and use the same screws and glue. Both are cellular PVC. Veranda is cheaper for standard trim boards. Azek offers more options, e.g. corner boards and bead board. (A photo is attached.)
(8) Both Veranda and Azek are available with a smooth or a woodgrain / 'frontier' surface, to match your house. You can order different versions from your Home Depot's Customer Service or Pro Desk.
(9) Store the trim flat and covered at your home or job site, to avoid distortions if they get hot in the summer sun, and stains from leaves, etc.
I hope this detail is helpful.
Mark