Question: What happens if your spark plug gap is too small?

Will carbon build up?

If so, can a frequent wire brush clean solve the problem?

Can be a potential problem but unlikely. It has more to do with the amount of spark required for proper ignition. Just like a big turbo motor can have issues with to much gap and the boost blowing the spark out. If you change to the turbo gap which i think is .8 and the car is tuned to run on the standard 1.1 you will have some dettination issues. Chiptorque avdised me not to do it, they said if i was desperate for a colder plug to try and gap it to 1.1 but they also said this will cause problems in the plug itself.

A wire brush maybe ok for cleaning near the centre and outer electrode but it won't clean any buildup down in the ceramic section surrounding the centre electrode. Only a beed blaster or spark plug cleaning machine can clean that section.

I use a Champion spark plug cleaner at work which beed blast, air blast and then a visual check of the plug is done. Gap is checked and then plug tested at 100psi (lookin for nice blue spark).

I've seen cheaper versions in autoparts store that connect to an air compressor (mini beed blaster).

If you're adjusting electrode gap, be gentle and don't hit it hard as this can fracture the outer electrode. The outer can detach and lodge in your engine.

The gapping tool at work is just like a mini vise which you slowly wind in. At home I just use my bench vise with soft jaws and adjust slowly. All adjustments are done on conventional plugs and some extra research may be needed for other plug types (Iridium etc).