x3
if one has a working horn and wants to blast it, stuff an old sock or shop rag into the opening, far enough in so the new paint will be visible, but the internals will be protected.
this may, or may not work. just tossing it out there.

Good idea. Anything to keep the crap out of the horns will work. Any media blasting dust gets through the smallest openings so before blasting. ANYTHING to make sure it is SEALED. Duct tape is generally pretty resistant as long as one does not BLAST DIRECTLY at it. Do keep in mind with horns they are two pieces with a gasket, so I would either avoid the area they join together or blast the edges at an angle

Can't you burn out the horn if you direct wire it to a battery? I thought the relay reduced power (amps?) to the horns so it didn't overheat the thin internal windings of the horn? Maybe that is only the road runner horns?

when testing a horn, i hook up a jumper wire to the horn's spade connector, touch the horn or it's mounting bracket [made sure there is clean material, no paint or rust] to the negative post of the battery, and just "tap" the jumper wire to the positive battery post.
if the horn is good, you will get a "beep", or high or low note sound from the horn.
i do not hold the jumper for a long time on the positive battery post.

No, the purpose of the relay is to handle the current of the horns that the horn switch can't.

if one looks at the wire on the horn button contact on the steering wheel, it is usually 18ga. sending only a ground path to the relay.
the wire[s] connecting to the horn[s] from the relay is usually 14ga.
quite a difference in amp carrying ability.