Superficial scratches and small gelcoat damage are quite doable yourself: a repair kit costs twenty to fifty euros and with patience the result is good. If the damage goes through the laminate, sits below the waterline, or the colour has to match exactly, doing it yourself usually works out more expensive, because it has to be redone.
Where doing it yourself works well
A scratch you can feel with a fingernail but which does not break through the gelcoat is DIY work. The same goes for a dull hull: polishing is labour, not craft, and it saves hundreds of euros in hourly rates.
Small gelcoat damage above the waterline is quite doable too. There are ready-made repair kits with gelcoat, hardener and abrasive paper, and the suppliers' instructions are generally accurate.
Where it goes wrong
The most common mistake is grinding away too little. A repair only bonds if the damaged material has been tapered back; the rule of thumb is a taper of roughly eight times the layer thickness on each side. On a hull of six millimetres that is five centimetres per side. Fill only the crack and you are sticking on a plaster that comes off within a season.
The second mistake is colour. Gelcoat is mixed to order and your hull has had years of sun. White from a kit is not your white. On the aft deck nobody notices; on a side deck at eye level they do.
The third is the weather. Polyester and epoxy need a minimum temperature to cure fully. A repair in October on a cold jetty does not cure through, stays tacky and has to come off.
The line, concretely
Have it done as soon as one of these four applies: the damage goes through the gelcoat into the laminate, it is below the waterline, it is in a load-bearing part such as around the keel bolts or the mast step, or an insurer is involved.
That last one is not a technical reason but a practical one: a repair you have carried out yourself makes assessing the damage afterwards difficult and can get in the way of a payout.
The sum that often gets forgotten
Doing it yourself costs materials plus time. A structural repair of half a square metre soon takes two weekends, including curing time between layers. Count those weekends before you make the comparison.
And count the chance of having to do it again. A failed repair has to come off before a professional can start, and that is extra work you would not otherwise have had.