It depends on how much too wide, why they are too wide, what wood the fingerboard is, what you are trying to accomplish, how important the instrument is . . . It's not a question with a single answer.
The current issue is with an older Yamaha with rosewood board; it's been fretted before, probably more than once. The rosewood isn't soft (particularly) but the slots are at least .027 and some are wider than that. I'm just trying to get new frets into it so it'll be ready by Xmas. My current fretwire has a .022" to .023" tang, so it doesn't want to hold.
Use wire with a fatter tang if you can, crimp the snot out of it,and maybe put a drop of thick superglue at each end of the slots and clamp the frets lightly if you have to. Good luck!
There is at least one other alternative. You can epoxy those frets in the slots. I'm not a big fan of this method, but I've had to refret several Stelling Banjos and a mandolin that were installed with epoxy at the factory. You'd probably have to widen the slots more than they are to use this method. It is a fairly simple operation but cleaning the excess glue off takes some time.
Ronnie Nichols
If you use epoxy as we did at Stelling, when you have the fretboard ready to fret, fill the fret slots with very fine wood dust. Then use a non-silicone wax to liberally wax the board and a little on the sides of the fingerboard. Now take your airhose and blow the sawdust out of the fret slots. This gives you an epoxy resistent fingerboard while leaving the slots clean and free of wax. The epoxy still has a little clean up, but is really no problem.
David
I did this on my second build (seen in my avatar) because I cut my fret slots to wide in a nice Ziricote board. Worked very well and was pretty simple to do.
I still sometimes use epoxy fretting, especially in a case like this. If the neck is stiff and/or has a truss rod it should work fine. I would use the widest fretwire I have, crimp it as close as I can and epoxy and clamp it in. The epoxy technique has been well documents in many places. I routinely used it back in the 80's when the "Teeter method" held sway. I still find it useful in problematic situations.
When I use superglue, with a clamp, I swab the edge with accelerator....A Q tip will do well. Yet, still, I find it best to wait a bit before moving to the next fret...Realy bad slots that are wider on the edges than in the middle seem to need plenty of time to properly set..Give them a couple minutes....And, remember, have a small fan going while you work... Move those vapors!!! I had to go to the hospital to stop a nosebleed that I truly believe was caused by superglue vapors.. I had done a couple refrets, and I allways superglue my frets because it works!! But I had also done a major finish build up project..An S.G. that someone put something under a Bigsby that was mounted on the guitar..It ate away little chunks of the finish, Alot in fact....I used one of those cherry Gibson pens to get the color close, and then used superglue to fill the many divots..I probably went through abouy 3 of those Stew Mac pippettes to fill it out...Then I sanded it flush, was able to polish it out...You had to look at it hard to see the fix...That worked well...NOW>>>superglue releases the same amount of toxic crappola when you sand it as well.. It activates the bad guys again...It was soon after that that the nosebleeds began. I know I didn't have proper ventilation, and was working while pretty much hovering over the job.,, Too much fumage...Too little moving air...Had to shove things ( plural) up my shnozz to stop it...Took a couple days....Read the label!...Damage to mucus membranes...I learned the hard way.. Remember, besides Firefighters, Law enforcement, Military, and Bully teasers, Guitar repair is the 5th most dangerous proffesion!!! Be smart!!, But still superglue works great....
just a new thought on this....epxoy mixed with dust...follow me... a 22 feeler gauge waxed and placed in the slot. a quick cure epoxy... the waxed feeler gauge can come out easy, and would give you a straght shot at a proper slot recut...