Hi guys,

The action on my guitar is quite low, so when I play with a slide the sound I get is very scratchy due to the strings being pressed against the frets. This happens even if i use the slide gently - it's much more noticeable playing lower down the neck.

Is there some little trick I can use to temporarily raise the action? I want to do this but without muting the open strings, as I like to play in open g and so would like to be able to return to the open chord.

The problem with slide guitar is typically you need a high action to do it effectively. If you still want to play open strings like you said then it's pretty much not possible. You're going to need to raise your action unfortunately.
A second guitar just for that purpose may be the only real answer.
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ive seen guys stick a pencil under the strings behind the first fret to raise action for slide. however there are two huge draw backs to this:
cant play open strings.

you dont get used to proper muting, because the strings are already sort of muted.

if you want to take slide seriously, get another guitar or raise your action.
the action doesnt have to be sooo high that normal playing is impossible though.
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Chea_man is the best.
As Chea-man says. Getting a guitar set up for slide that also allows for fingering is a balancing act. Most acoustic guitars manage pretty well because the string tension is considerably higher than on an electric, and the normal action higher as well.

Fortunately, a dedicated slide guitar doesn't have to be very good... A lot of players have turned ratty second-hand instruments into good slide guitars... You often want a rather nasty sound.