i have been building 1:6 bikes for some years now and the biggest challenge is chrome. Plated sprue parts are difficult or impossible to touch up. Furthermore if you have two halves like an exhaust muffler then there is the problem of dealing with the joints sink holes or ejector pin marks. Al-Clad does a good job but it's very difficult to get right and is totally unforiving of any errors
recently i think practice might be yielding some more realistic results and at least this way there are no ejector pin or sink hole marks
