By David S. Lawyer. mailto:firstname.lastname@example.org
While there are tools made for resurfacing brass faucet valve seats, their cutting heads are usually not large enough for use on sprinkler valve seats. The solution is to make your own tool or use some type of fine grit grinding tool (if you have one). In most cases, little of the seat brass needs to be removed so one can turn the tool by ones fingers and often complete the job after several turns back and forth. If one uses a power tool, there is the risk of removing too much brass, unless you go at a very slow speed for only a second or two.
To make a hand tool for surfacing the seat of a 1" sprinker valve (other sizes are done similarly) use say 600 grit wet-or-dry sandpaper. Finer grit is OK. Then find a long machine screw (say 3/8" diameter) to mount some washers on. Use a large diameter fender washer (at least the diameter of the rubber washer) to mount the sandpaper on. Lay this fender washer on the sheet of sandpaper and use fingernail-trimming scissors to cut out a sandpaper disk of the same size. Then with a pencil, trace a circle on the sanding disk thru the center hole of the washer and cut that out so as to make a center hole in the sanding disk, the same diameter as the machine screw.
Then gather all the parts and assemble your homemade tool. The parts: Machine screw a few in. long; sandpaper disk; fender washer, 3@ standard washers, 2 nuts. Put them on the machine screw in the following order: nut, standard washer, fender washer, sandpaper disk, 2@ standard washers, nut. Tighten the nuts, put the tool into the valve with the sandpaper centered on the seat, apply a little pressure on the seat and twist back and forth, trying to keep the sanding disk level on the seat. Be sure the sanding disk is wet. After the first few twists pull up the tool and look at the seat. You are about done when the seat surface shines all the way around and is wider than a knife edge. If it's too wide, it's more apt to get sediment, etc trapped between the seat surface and the rubber washer, resulting in a leak.
The tool will be a little easier to twist by hand if you have the hex head of the machine screw right next to the sanding disk. This is opposite to the assembly sequence described above and you only need one nut, but that nut will take longer to screw on since it has to travel the entire length of the machine screw. Instead of a machine screw, I used the threaded shaft of a faucet seat surfacing tool
The reason for using two standard diameter washers next to each other is to use them to keep the tool somewhat aligned with the seat hole (these washers fit into the hole in the seat). If the sprinkler valve uses a freely rotating rubber washer holder, one may use this for mounting the sanding disk (instead of a machine screw).
While it may take longer to resurface the seat than it takes to just replace the entire valve, the advantages are that it both saves money and you may wind up with a better quality valve, since the old ones made in the USA are usually of higher quality than the imports made today.