I've really no idea what a stx38 is - assume its a garden machine? Anyway, no matter. The wheels are likely to be held by a key which often rusts in - you could try some WD40 spray or similar. However, I use a product called 'Tyreweld' which, when injected through the valve, seals existing puntures and also protects against new ones. It seals holes up to around 5mm.
John
January 2008