I should start by saying I am not a qualified health professional, but here is some factual information that I have picked up here and there.

Many sex toys are made from materials that seem to have smooth and uninterrupted surfaces, but are actually porous. This includes most plastics, flexible rubbery materials, ceramics (unless glazed) and metals including stainless steel. Most protective, decorative or cosmetic coatings used on domestic objects made from steel or other metals are also permeable to bodily fluids, bacteria and viruses.

Bodily fluids can be absorbed into all these materials and can transmit STDs to another user of the same toy. Normal hygiene precautions such as rinsing the item with disinfectant are a good idea, but may not be totally effective. Dildos and other similar sex toys, if used by multiple partners, should be fitted with a condom before each use.

Where a regular partner has HIV, I have heard that there is a high incidence of transmission due to frequent sex, even if a condom is used. Body fluids only need to come in contact with a mucous membrane or a break in the skin caused by any reason. Where a partner has an STD and a condom would normally provide protection, the probability of transmission is multiplied by the number of times you have sex, and where this number is greater than 12 the probability of transmission becomes significant. The only really safe practice is not to have sex with an infected person.

Whips, canes and other implements used for punishment are also permeable to bodily fluids and any microorganisms they may contain. It is unsafe to use the same implement on multiple subjects if any of them are carrying any kind of STD that can be transmitted through the skin. Drawing blood is particularly dangerous if the subject might be infected, as blood only needs to come in contact with a break in the skin or a mucous membrane for transmission of HIV or another STD to occur. Blood or body fluids absorbed into leather or another organic material can transmit an STD to another person who is punished with the same implement. The only safe practice is for a particular implement to be used only on a single individual, or of course, to avoid corporal punishment altogether.

Painting a wooden object is not sufficient to make it impermeable. Marine varnish is better than paint, but in addition to several coats of varnish I would still advise scrupulous disinfection (and use of a condom where appropriate).

Ropes, cords, straps, cuffs and other bondage equipment can be a vehicle for transmitting an STD. It is important to ensure that people using this equipment do not have external lesions such as warts that can be picked up by the leather and transmitted. Parts of the boby exposed to such items can be wrapped with a protective layer such as cling film to prevent direct contact with the skin. This is especially important in clubs and other places where the same equipment is used by many members.