I'm biting the bullet and dropping (to me) a substantial amount of cash on a high-end used fretless. I know there are TB members who can buy and sell me, but I'm a blue collar fella, so... Anywho, it's currently strung with rounds. I don't know what brand, gauge, etc. I worry about this, because I've witnessed a fair amount of vitriol regarding rounds on a fretless. Should I get flats? What brand? What gauge? (It's a fiver). What the heck are half rounds? Tape wounds? HELP!!!
The deal is this. On a fretless bass, rounds will eventually chew grooves into the fingerboard even if it's ebony and even faster if it's softer like rosewood or maple. Stainless steel rounds are the worst, nickel less so. When I got my Alembic 6er fretless I noticed the strings (which were supposed to be "pressure-wounds" immediately chewing the fingerboard. I FREAKED OUT!
Well, to stop the chews you need strings smooth on the outside. Standard are flat-wounds. These are strings wrapped with flat metal tape and are quite smooth. They are also the dullest sounding strings of all the types. They do have a certain appeal, to many people though. The brightest you can go and still have smooth strings are ground-wounds. GHS makes them and calls them "brite flats" and D'addario makes them and used to call theirs "half-rounds". These are round-wound strings (which are strings wound with plain old round wire) that have been ground smooth on the outside. These are my favorite and "standard" fretless string because they don't chew grooves in the fingerboard and yet are reasonably bright sounding if you want that. My G&L fretless has D'addario grounds on it. My Alembic has GHS brite flats on it. Love 'em.
Another thing to consider is nylon wrapped strings like Fender or LaBella sell. I use these on my Carvin AC50 semi-hollow body fretless. They are very smooth and nice strings with a great feel and a decent sound. Worth trying on your fretless bass. They are wrapped with nylon tape on the outside and hence are usually black with a plastic slippery feel. Since they are plastic covered they don't chew.
And of course, lastly there are round wounds. Nothing is as bright and will sound like that. Hence, some people use them on their fretless because they want the tone. If you go that way you'll just have to figure that one day you'll have to take your bass and have the fingerboard leveled to take the grooves out. In this case you just have to consider that the "cost of doing business" to get the tone you want. It's not "vitriol", it's just how it is. How long you go between operations depends on how you play and the bass.
A compromise between rounds and grounds are pressure-wounds. As it turns out my Alembic came with rounds though they called them pressure wounds. GHS and Ken Smith and few others make true pressure wounds. These are round-wound strings they run through rollers to sort of smooth the outside. They still chew a bit but much less than rounds. I've used them, but don't like them because I think they have a strange tone, but that is for you, not me, to decide.
My advice would be to start with a set of ground-wound strings and then as time goes on try a set of the other types every now and then to see what you think. There are also TI flats which are said to be flexible and very nice but are quite expensive so I've never tried them.
As to gauges I don't worry much. I just buy a standard string set (medium) and have at it. If one wants to get down to tiny nuances in strings there are places online where you can buy strings in various gauges etc. But I try not to go that far. When you get into 5 and 6 string sets availability starts to get scarce (especially locally) and prices go up! Hence, I usually just go with standard sets I can easily find. I'd decide what type of string is best on your "new" bass before I started fooling with gauges.
Good luck.
