@Razvan: you will have a hard time finding a cheap ND filter with the same "darkness" as a piece of welding glass, and normally welding glass is quite good quality considering what they are made for.
i just checked with a welding glass and got 1/1000sec, 1/3stop overexposed without the glass and 15sec 2/3stop underexposed, so that equals around a 14 stop reduction.
Canon 1D MK3, 5D MK3
Canon 50mm 1.4 USM, 100mm F2.8 USM, 70-200 F2.8 L
16-35 F2.8 L
Sigma 300mm F2.8
Canon Pixma PRO-9000 MKII
and a 17-40 F4L in a hundred little pieces