Why the bottom gets greasy in the first place
At the base of a guinea pig's spine, roughly where a tail would sit, is a grease gland that produces a waxy, sometimes sticky secretion. In boars it is often more active, and the fur around it can clump into a dark, greasy patch that picks up bedding and droppings. A local build-up by itself is a grooming issue, not dirtiness or neglect, and it does not call for bathing the whole animal. Guinea pigs are not natural swimmers, they chill quickly when wet through, and a full bath strips the coat and stresses most pigs far beyond what the problem requires. The job here is small and targeted: soften, wash and dry the rear only.
Setting up before you lift your pig
Gather everything first so your guinea pig is never left waiting in water while you search a cupboard. You need a shallow bowl or a sink lined with a folded towel for grip, lukewarm water no deeper than belly height for the rear end only, a soft cloth, a small amount of gentle wash made for small animals, and two dry towels. Work in a warm, draught-free room and keep the session short. A second person to hold the pig calmly against their chest while you wash is a real help with a wriggler, because a guinea pig who feels secure panics less and the wash finishes faster.
If the grease is thick, start dry. Rest the pig on a towel on your lap and press a warm, damp cloth against the patch for a minute or two to soften it, then ease loosened debris away with your fingers. Matted fur that will not release should be trimmed carefully rather than pulled, keeping the skin flat and cutting well away from it, or left for a groomer or vet if it sits tight against the skin.
Washing just the rear, step by step
Lower only the pig's bottom into the shallow water and support its front end on the towel at the bowl's edge so its head, chest and front feet stay dry. Work the warmed water through the greasy fur with your fingertips, add the smallest amount of wash, and massage gently until the waxy residue loosens. Never dunk the pig, never pour water over its head, and stop for a cuddle break if it becomes frantic; a stressed pig can be finished with the damp-cloth method instead. Rinse the washed area with clean lukewarm water until no slippery residue remains, because leftover wash left on skin can itch once it dries.
Lift the pig straight into a dry towel, wrap the rear warmly, and pat and fluff the fur dry. A pig who still feels damp underneath should stay on your lap in the warm room until fully dry before going back to the enclosure, especially in cooler weather or for an older pig who feels the cold sooner.
When greasy becomes a vet visit
See a veterinarian if the skin under the grease is red, broken, scabbed or sore, if there is a lump at the gland, a strong sour smell that returns within days, or fur loss spreading beyond the patch. A bottom that is soiled with soft droppings rather than gland grease also deserves attention, because that points to diet or health rather than grooming, and a pig who has stopped cleaning itself may be sore, overweight or unwell in a way a bath cannot fix. Describe whether the build-up is waxy and localised or loose soiling across the rear, since that distinction steers the appointment.
For the ordinary case, a wash every so often as the gland demands it is enough, and logging the date in KinsLe Care Teams means a sitter or partner can check when the last rear wash happened instead of bathing again out of guesswork.