Hi @rhodes,
A warm welcome to the Bunnings Workshop community and thank you for your question.
Typically, paint or primers won't adhere well to silicone. This is why silicone is often coloured and is applied after painting.
If it is an exterior gap filler as opposed to silicone, then it can be painted without any issues, but if it is silicone then paint will not stick to it regardless of whether you use a primer or not.
Exterior paint such as Dulux Weathershield, Taubmans Sunproof and British Paints 4 Seasons are self-priming, so there is no real need for a primer if you are using an exterior paint such as these.
Let me know if you have any further questions.
Jacob
