The grease will leech between the boot and the driveshaft over time. It's just one of those things you need to learn to live with. Some vehicles will never weep, other driveshafts will weep constantly no matter what you try.

Remember that the purpose of the boots is to keep dirt (etc) out of the CV joint, not to keep the grease in.

I'd probably use some degreaser applied to a rag and wiped over the surface before installing the clamp.

After replacing hundreds upon hundreds of these things over the years (I'm a qualified mechanic), it's my observation that both clamps and zip ties work equally well. If using zip ties, use 2 and install them with the "ratchet" on opposite sides of the shaft. Not for balance - the only real "loose point" in the tie is immediately under the ratchet. Offset them to prevent dust ingress.

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Cheers,

Anthony